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!!

Command/Indirect.hs

@@ -13,6 +13,7 @@ import qualified Git
 import qualified Git.Command
 import qualified Git.LsFiles
 import Config
+import qualified Annex
 import Annex.Direct
 import Annex.Content
 import Annex.CatFile
@@ -27,10 +28,12 @@ seek = [withNothing start]
 
 start :: CommandStart
 start = ifM isDirect
-	( ifM probeCrippledFileSystem
-		( error "This repository seems to be on a crippled filesystem, you must use direct mode."
-		, next perform
-		)
+	( do
+		unlessM (coreSymlinks <$> Annex.getGitConfig) $
+			error "Git is configured to not use symlinks, so you must use direct mode."
+		whenM probeCrippledFileSystem $
+			error "This repository seems to be on a crippled filesystem, you must use direct mode."
+		next perform
 	, stop
 	)