CLI: make gpg binary used by libgmime configurable.

CLI: make gpg binary used by libgmime configurable.

Previously we set up a way for the top level notmuch command to choose
which gpg binary was invoked by libgmime. In this commit we add the
(mostly boilerplate) code to allow the notmuch-config command to read
and write this path, and use it in the appropriate struct.

Update tests for new default variable