Merge branch 'jk/gpg-interface-cleanup'

A new run-command API function pipe_command() is introduced to
sanely feed data to the standard input while capturing data from
the standard output and the standard error of an external process,
which is cumbersome to hand-roll correctly without deadlocking.

The codepath to sign data in a prepared buffer with GPG has been
updated to use this API to read from the status-fd to check for
errors (instead of relying on GPG's exit status).

* jk/gpg-interface-cleanup:
  gpg-interface: check gpg signature creation status
  sign_buffer: use pipe_command
  verify_signed_buffer: use pipe_command
  run-command: add pipe_command helper
  verify_signed_buffer: use tempfile object
  verify_signed_buffer: drop pbuf variable
  gpg-interface: use child_process.args