crippled filesystem support, probing and initial support

git annex init probes for crippled filesystems, and sets direct mode, as
well as `annex.crippledfilesystem`.

Avoid manipulating permissions of files on crippled filesystems.
That would likely cause an exception to be thrown.

Very basic support in Command.Add for cripped filesystems; avoids the lock
down entirely since doing it needs both permissions and hard links.
Will make this better soon.

Command/Fsck.hs

@@ -203,7 +203,8 @@ fixLink key file = do
 				showNote "fixing content location"
 				dir <- liftIO $ parentDir <$> absPath file
 				let content = absPathFrom dir have
-				liftIO $ allowWrite (parentDir content)
+				unlessM crippledFileSystem $
+					liftIO $ allowWrite (parentDir content)
 				moveAnnex key content
 
 		showNote "fixing link"