* trustdb.h, trustdb.c (mark_usable_uid_certs): Add flags for the
no-pubkey and chosen revocation cases.  (clean_uid): New function to
clean a user ID of unusable (as defined by mark_usable_uid_certs)
certs.
* keyedit.c (keyedit_menu, menu_clean_uids): Call it here for new
"clean" command that removes unusable sigs from a key.