core: Extend gpgme_user_id_t with uidhash member.

* src/gpgme.h.in (struct _gpgme_user_id): Add field 'uidhash'.
* src/key.c (gpgme_key_unref): Free it.
* src/keylist.c (keylist_colon_handler): Set it.
* tests/run-keylist.c (main): Print it.

The uidhash value is part of gpg's output since the year 2005. This
now adds support to gpgme. The application for uidhash is to select
a user id in an edit interactor: Instead of giving the number of the
user id, the uidhash value can be be used to avoid tracking the user
id numbers.

• Signed-off-by: Werner Koch <wk@gnupg.org>