Which had some weird inheriting of ssh FDs by sshd.

Bug was introduced in git-annex version 7.20200202.7.

Bug was introduced as part of a different FD leak fix in version 6.20160318.

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That removed the list function. This new implementation appears to
actually be more efficient anyway, since it avoids toList.

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for consistency with external backend protocol

This is groundwork for external backends, but also makes sense to keep
this information with the rest of a Backend's implementation.

Also, removed isVerifiable. I noticed that the same information is
encoded by whether a Backend implements verifyKeyContent or not.

And add a test case for that.

This certianly loses some of the 2x performance improvement in file
seeking that seekFilteredKeys led to, because now it has to stat the
worktree files again. Without benchmarking, I expect there will still be
a sizable improvement, and also the git-annex branch precaching that
seekFilteredKeys can do will still be a win of its approach.

Also worth noting that lookupKey, when the file DNE, check if it's in an
adjusted branch with hidden files, and if so, finds the key for the
file anyway. That was intended to make git-annex sync --content be able
to process those files, but a side effect was that, when a file was
deleted but the deletion not yet staged, git-annex commands used to
still list it. That was actually a bug. This commit fixes that bug too.
(git-annex sync --content on such a branch does not use seekFilteredKeys
so was not affected by the reversion or by this behavior change)

This commit was sponsored by Jake Vosloo on Patreon.