merge: boolify functions

These functions returned -1 on failure but are essentially boolean.
Convert them to use false/true.
 * check_all_msg()
 * check_msg()
 * compare_certificates()
 * compare_stat()
 * crypt_key_is_valid()
 * is_bound()
 * line_compare()
 * mutt_check_charset()
 * rfc822_valid_msgid()
 * smtp_code()
 * url_parse_query()
 * user_in_addr()