speculatively use remote's configured chunk size as a fallback

speculatively use remote's configured chunk size as a fallback

When a special remote has chunking enabled, but no chunk sizes are
recorded (or the recorded ones are not found), speculatively try chunks
using the configured chunk size.

This makes eg, git-annex fsck --from remote be able to fix up the
location log of a file that the git-annex branch does not indicate is
stored on the remote.

Note that fsck does *not* fix up the chunk log to indicate the chunk
size. So, changing the chunk config of the remote after that will still
prevent accessing the chunks stored on it. Maybe fsck should, but I
wanted to start with this and see if it's needed.