proxying special remotes

proxying special remotes

This is early, but already working for CHECKPRESENT.

However, when the special remote throws an exception on checkPresent,
this happens:

[2024-06-28 13:22:18.520884287] (P2P.IO) [ThreadId 4] P2P > ERROR directory /home/joey/tmp/bench2/dir is not accessible
[2024-06-28 13:22:18.521053135] (P2P.IO) [ThreadId 4] P2P < ERROR expected SUCCESS or FAILURE
git-annex: client error: expected SUCCESS or FAILURE
(fixing location log) p2pstdio: 1 failed

  ** Based on the location log, x
  ** was expected to be present, but its content is missing.
failed