* trustdb.h, trustdb.c (clean_subkeys_from_key): New.  Walk through
the subkeys on a key, and mark any that aren't usable for deletion.
Note that a signing subkey is never marked for deletion since these
keys are still useful after expiration or revocation.
* keyedit.c (menu_clean_subkeys_from_key): New function to call
clean_subkeys_from_key() on a key.  Note that the strings here are not
marked for translation yet.  The UI is still in flux, and there is no
point in annoying the translators twice.  (keyedit_menu): Call it here
as part of the "clean" command.