* trustdb.h, trustdb.c (clean_uids_from_key): New.  Walk through the
user IDs on a key, and mark any that aren't valid for deletion.
* keyedit.c (menu_clean_uids_from_key): New function to call
clean_uids_from_key() on a key.  (keyedit_menu): Call it from here as
part of the "clean" command.