fully support core.symlinks=false in all relevant symlink handling code

Refactored annex link code into nice clean new library.

Audited and dealt with calls to createSymbolicLink.
Remaining calls are all safe, because:

Annex/Link.hs:  ( liftIO $ createSymbolicLink linktarget file
  only when core.symlinks=true
Assistant/WebApp/Configurators/Local.hs:                createSymbolicLink link link
  test if symlinks can be made
Command/Fix.hs: liftIO $ createSymbolicLink link file
  command only works in indirect mode
Command/FromKey.hs:     liftIO $ createSymbolicLink link file
  command only works in indirect mode
Command/Indirect.hs:                    liftIO $ createSymbolicLink l f
  refuses to run if core.symlinks=false
Init.hs:                createSymbolicLink f f2
  test if symlinks can be made
Remote/Directory.hs:    go [file] = catchBoolIO $ createSymbolicLink file f >> return True
  fast key linking; catches failure to make symlink and falls back to copy
Remote/Git.hs:          liftIO $ catchBoolIO $ createSymbolicLink loc file >> return True
  ditto
Upgrade/V1.hs:                          liftIO $ createSymbolicLink link f
  v1 repos could not be on a filesystem w/o symlinks

Audited and dealt with calls to readSymbolicLink.
Remaining calls are all safe, because:

Annex/Link.hs:		( liftIO $ catchMaybeIO $ readSymbolicLink file
  only when core.symlinks=true
Assistant/Threads/Watcher.hs:		ifM ((==) (Just link) <$> liftIO (catchMaybeIO $ readSymbolicLink file))
  code that fixes real symlinks when inotify sees them
  It's ok to not fix psdueo-symlinks.
Assistant/Threads/Watcher.hs:		mlink <- liftIO (catchMaybeIO $ readSymbolicLink file)
  ditto
Command/Fix.hs:	stopUnless ((/=) (Just link) <$> liftIO (catchMaybeIO $ readSymbolicLink file)) $ do
  command only works in indirect mode
Upgrade/V1.hs:	getsymlink = takeFileName <$> readSymbolicLink file
  v1 repos could not be on a filesystem w/o symlinks

Audited and dealt with calls to isSymbolicLink.
(Typically used with getSymbolicLinkStatus, but that is just used because
getFileStatus is not as robust; it also works on pseudolinks.)
Remaining calls are all safe, because:

Assistant/Threads/SanityChecker.hs:                             | isSymbolicLink s -> addsymlink file ms
  only handles staging of symlinks that were somehow not staged
  (might need to be updated to support pseudolinks, but this is
  only a belt-and-suspenders check anyway, and I've never seen the code run)
Command/Add.hs:         if isSymbolicLink s || not (isRegularFile s)
  avoids adding symlinks to the annex, so not relevant
Command/Indirect.hs:                            | isSymbolicLink s -> void $ flip whenAnnexed f $
  only allowed on systems that support symlinks
Command/Indirect.hs:            whenM (liftIO $ not . isSymbolicLink <$> getSymbolicLinkStatus f) $ do
  ditto
Seek.hs:notSymlink f = liftIO $ not . isSymbolicLink <$> getSymbolicLinkStatus f
  used to find unlocked files, only relevant in indirect mode
Utility/FSEvents.hs:                    | Files.isSymbolicLink s = runhook addSymlinkHook $ Just s
Utility/FSEvents.hs:                                            | Files.isSymbolicLink s ->
Utility/INotify.hs:                             | Files.isSymbolicLink s ->
Utility/INotify.hs:                     checkfiletype Files.isSymbolicLink addSymlinkHook f
Utility/Kqueue.hs:              | Files.isSymbolicLink s = callhook addSymlinkHook (Just s) change
  all above are lower-level, not relevant

Audited and dealt with calls to isSymLink.
Remaining calls are all safe, because:

Annex/Direct.hs:			| isSymLink (getmode item) =
  This is looking at git diff-tree objects, not files on disk
Command/Unused.hs:		| isSymLink (LsTree.mode l) = do
  This is looking at git ls-tree, not file on disk
Utility/FileMode.hs:isSymLink :: FileMode -> Bool
Utility/FileMode.hs:isSymLink = checkMode symbolicLinkMode
  low-level

Done!!

Backend.hs

@@ -17,12 +17,11 @@ module Backend (
 	maybeLookupBackendName
 ) where
 
-import System.Posix.Files
-
 import Common.Annex
 import qualified Annex
 import Annex.CheckAttr
 import Annex.CatFile
+import Annex.Link
 import Types.Key
 import Types.KeySource
 import qualified Types.Backend as B
@@ -77,15 +76,12 @@ genKey' (b:bs) source = do
 		| otherwise = c
 
 {- Looks up the key and backend corresponding to an annexed file,
- - by examining what the file symlinks to.
+ - by examining what the file links to.
  -
- - In direct mode, there is often no symlink on disk, in which case
- - the symlink is looked up in git instead. However, a real symlink
+ - In direct mode, there is often no link on disk, in which case
+ - the symlink is looked up in git instead. However, a real link
  - on disk still takes precedence over what was committed to git in direct
  - mode.
- -
- - On a filesystem that does not support symlinks, git will instead store
- - the symlink target in a regular file.
  -}
 lookupFile :: FilePath -> Annex (Maybe (Key, Backend))
 lookupFile file = do
@@ -107,35 +103,6 @@ lookupFile file = do
 					" (unknown backend " ++ bname ++ ")"
 				return Nothing
 
-{- Checks if a file is a symlink to a key.
- -
- - On a filesystem that does not support symlinks, git will instead store
- - the symlink target in a regular file. (Only look at first 8k of file,
- - more than enough for any symlink target.)
- -}
-isAnnexLink :: FilePath -> Annex (Maybe Key)
-isAnnexLink file = maybe Nothing makekey <$> gettarget
-  where
-	gettarget = ifM (coreSymlinks <$> Annex.getGitConfig)
-		( liftIO $ catchMaybeIO $ readSymbolicLink file
-		, liftIO $ catchMaybeIO $ take 8192 <$> readFile file
-		)
-	makekey l
-		| isLinkToAnnex l = fileKey $ takeFileName l
-		| otherwise = Nothing
-
-{- Creates a symlink on disk.
- -
- - On a filesystem that does not support symlinks, writes the link target
- - to a file. Note that git will only treat the file as a symlink if
- - it's staged as such.
- -}
-makeAnnexLink :: String -> FilePath -> Annex ()
-makeAnnexLink linktarget file = ifM (coreSymlinks <$> Annex.getGitConfig)
-	( liftIO $ createSymbolicLink linktarget file
-	, liftIO $ writeFile file linktarget
-	)
-
 {- Looks up the backend that should be used for a file.
  - That can be configured on a per-file basis in the gitattributes file. -}
 chooseBackend :: FilePath -> Annex (Maybe Backend)