merge: boolify functions

These functions all returned 0/1 on failure/success.
Convert them to use false/true.
 * add_quote_suffix()
 * addrcmp()
 * addresses_use_unicode()
 * address_uses_unicode()
 * check_boundary()
 * check_certificate_cache()
 * check_certificate_expiration()
 * check_for_mailing_list()
 * check_idn()
 * cmd_queue_full()
 * compare_flags()
 * count_body_parts_check()
 * create_hcache_dir()
 * crypt_is_numerical_keyid()
 * feature_enabled()
 * get_initials()
 * hostname_match()
 * imap_has_flag()
 * is_context_available()
 * is_day_name()
 * is_display_corrupting_utf8()
 * link_threads()
 * maildir_add_to_context()
 * match_reference()
 * mbox_has_new()
 * mh_valid_message()
 * mutt_addr_is_user()
 * mutt_comp_can_append()
 * mutt_comp_can_read()
 * mutt_group_match()
 * mutt_limit_current_thread()
 * mutt_matches_list()
 * mutt_match_spam_list()
 * mutt_needs_mailcap()
 * mutt_pop_list()
 * mutt_ts_capability()
 * perform_and()
 * perform_or()
 * pgpring_string_matches_hint()
 * pgp_use_gpg_agent()
 * prepare_sidebar()
 * select_next()
 * select_next_new()
 * select_prev()
 * select_prev_new()
 * string_is_address()