add lockContentShared

Also, rename lockContent to lockContentExclusive

inAnnexSafe should perhaps be eliminated, and instead use
`lockContentShared inAnnex`. However, I'm waiting on that, as there are
only 2 call sites for inAnnexSafe and it's fiddly.

Utility/LockPool/Posix.hs

@@ -6,6 +6,7 @@
  -}
 
 module Utility.LockPool.Posix (
+	P.LockFile,
 	LockHandle,
 	lockShared,
 	lockExclusive,