add remote fsck interface

Currently only implemented for local git remotes. May try to add support
to git-annex-shell for ssh remotes later. Could concevably also be
supported by some special remote, although that seems unlikely.

Cronner user this when available, and when not falls back to
fsck --fast --from remote

git annex fsck --from does not itself use this interface.
To do so, I would need to pass --fast and all other options that influence
fsck on to the git annex fsck that it runs inside the remote. And that
seems like a lot of work for a result that would be no better than
cd remote; git annex fsck
This may need to be revisited if git-annex-shell gets support, since it
may be the case that the user cannot ssh to the server to run git-annex
fsck there, but can run git-annex-shell there.

This commit was sponsored by Damien Diederen.

Remote/WebDAV.hs

@@ -65,6 +65,7 @@ gen r u c gc = new <$> remoteCost gc expensiveRemoteCost
 			hasKey = checkPresent this,
 			hasKeyCheap = False,
 			whereisKey = Nothing,
+			remoteFsck = Nothing,
 			config = c,
 			repo = r,
 			gitconfig = gc,