use programPath consistently, not readProgramFile

Improve git-annex's ability to find the path to its program, especially
when it needs to run itself in another repo to upgrade it.

Some parts of the code used readProgramFile, probably because I forgot that
programPath exists.

I noticed this when a git-annex auto-upgrade failed because it was running
git-annex upgrade --autoonly, but the code to run git-annex used
readProgramFile, which happened to point to an older build of git-annex.

Annex/Multicast.hs

@@ -7,7 +7,7 @@
 
 module Annex.Multicast where
 
-import Config.Files
+import Annex.Path
 import Utility.Env
 import Utility.PartialPrelude
 
@@ -22,7 +22,7 @@ multicastReceiveEnv = "GIT_ANNEX_MULTICAST_RECEIVE"
 
 multicastCallbackEnv :: IO (FilePath, [(String, String)], Handle)
 multicastCallbackEnv = do
-	gitannex <- readProgramFile
+	gitannex <- programPath
 	-- This will even work on Windows
 	(rfd, wfd) <- createPipeFd
 	rh <- fdToHandle rfd