When invoking gpg as a backgrounded tool, it's important to let gpg
know that it is backgrounded, to avoid spurious prompts or other
breakage.
In particular, https://bugs.debian.org/913614 was a regression in
GnuPG which causes problems when importing keys without a terminal,
but gpg expects one.
Ensuring that notmuch-emacs always invokes gpg as a background process
should avoid some of these unnecessary failure.
Thanks to Justus Winter for finding this problem.
Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>