cli: crypto: tell gmime to use gpg-agent

cli: crypto: tell gmime to use gpg-agent

For decryption, we expect there to be a functioning gpg-agent, and we
want gpg to talk to it for any needed credentials. There's a gmime
function to declare that: g_mime_gpg_context_set_use_agent() [1], [2].
Start using it.

I had gpg-agent running, but gpg "use-agent" configuration option
disabled. This resulted in an error message from 'notmuch show':

  Failed to decrypt part: Canceled.

and json had this:

  "encstatus" : [ { "status" : "bad" } ]

One could argue the "use-agent" option should be enabled, but I'd like
to use the agent only as a last resort. I think that's irrelevant
though. There's a gmime function to declare what we expect, so we
should use it. Conveniently it also fixes the problem in a user
friendly way.

[1] http://git.gnome.org/browse/gmime/commit/?id=ed985397843a9da3745a8b5de3d1d652acd24724
[2] https://bugzilla.gnome.org/show_bug.cgi?id=651826