emacs: Add callback functions to crypto sigstatus button.

This adds two callback functions to the sigstatus button.  If the sig
status is "good", then clicking the button displays the output of "gpg
--list-keys" on the key fingerprint.  If the sigstatus is "bad", then
clicking the button will retrieve the key from the keyserver, and
redisplay the current buffer.

Thanks to David Bremner <bremner@unb.ca> for help with this.