[PATCH 0/7] gpg-interface cleanups

This started off with Michael's patch to sign_buffer, which is at the tip, and then me trying to address the possible deadlocks there and in verify_signed_buffer. While I was in the area, I took the opportunity to do a few cleanups.

It's unclear to me whether the deadlocks are possible in practice; see patch 5 for discussion. My guess is probably not, but the amount of code to support doing it right is not all that much. But if we don't like it, we can drop 4-6.

Patch 7 is still authored by Michael, but has been massaged greatly by me. I'll comment more directly on the changes there.

  [1/7]: gpg-interface: use child_process.args
  [2/7]: verify_signed_buffer: drop pbuf variable
  [3/7]: verify_signed_buffer: use tempfile object
  [4/7]: run-command: add pipe_command helper
  [5/7]: verify_signed_buffer: use pipe_command
  [6/7]: sign_buffer: use pipe_command
  [7/7]: gpg-interface: check gpg signature creation status