lib: Kill GPG agent when cleaning up tmp homedirs

lib: Kill GPG agent when cleaning up tmp homedirs

When a temporary directory is used for GPG operations, it's pretty clear
that the running agent will be useless after the directory is deleted.
Call the new `ot_gpgme_kill_agent ()` helper to kill gpg-agent rather
than leaving them it hanging around forever.

As it turns out, gnupg does have code to make gpg-agent automatically
exit when the homedir is removed (https://dev.gnupg.org/T2756), but
that's only available on gnupg 2.2 or newer. Possibly this code can be
dropped later when that's more widely deployed or users/distros have
been advised to backport the necessary changes.

Closes: #1799
Approved by: cgwalters