gpg: Add accessor & utility functions for pk->keyid and pk->main_keyid.

* g10/keydb.h (keyid_cmp): New function.
* g10/keyid.c (pk_keyid): New function.
(pk_main_keyid): New function.
(keyid_copy): New function.
(pk_keyid_str): New function.
* g10/packet.h (PKT_public_key): Update comments for main_keyid and
keyid.

Before accessing pk->keyid, it is necessary to call keyid_from_pk (pk,
NULL) to ensure that pk->keyid is valid. Because it is easy to forget
to do this, these accessor functions take care of it.

• Signed-off-by: Neal H. Walfield <neal@g10code.com>