fix a annex.pidlock issue

That made eg git-annex get of an unlocked file hang until the
annex.pidlocktimeout and then fail.

This fix should be fully thread safe no matter what else git-annex is
doing.

Only using runsGitAnnexChildProcess in the one place it's known to be a
problem. Could audit for all places where git-annex runs itself as a child
and add it to all of them, later.

Annex/LockPool/PosixOrPid.hs

@@ -18,6 +18,7 @@ module Annex.LockPool.PosixOrPid (
 	LockStatus(..),
 	getLockStatus,
 	checkSaneLock,
+	pidLockFile,
 ) where
 
 import Common