support being run by ssh as ssh-askpass replacement

To use, set GIT_ANNEX_SSHASKPASS to point to a fifo or regular file
(FIFO is better, avoids touching disk or multiple readers) that contains
the password. Then set SSH_ASKPASS=git-annex, and when ssh runs it, it will
tell ssh the password.

This is not yet used..

CmdLine/GitAnnex.hs

@@ -199,5 +199,11 @@ run args = do
 #ifdef WITH_EKG
 	_ <- forkServer "localhost" 4242
 #endif
-	maybe (dispatch True args cmds gitAnnexOptions [] header Git.CurrentRepo.get)
-		(runSshCaching args) =<< getEnv sshCachingEnv
+	go envmodes
+  where
+	go [] = dispatch True args cmds gitAnnexOptions [] header Git.CurrentRepo.get
+	go ((v, a):rest) = maybe (go rest) a =<< getEnv v
+	envmodes =
+		[ (sshCachingEnv, runSshCaching args)
+		, (sshAskPassEnv, runSshAskPass)
+		]