gpg: Make the get_pubkey_byname interface easier to understand.

* g10/keydb.h (enum get_pubkey_modes): New.
* g10/getkey.c (get_pubkey_byname): Repalce no_akl by a mode arg and
change all callers.

This change prepares the implementation of GET_PUBKEY_NO_LOCAL.

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