merge: upstream Buffer refactoring

 * Convert hcache fetch and store raw to use buffer pool
 * Convert mutt_parse_hook() to use buffer pool
 * Convert set/reset and source_rc to use buffers for paths
 * Convert main() to use buffers for paths
 * Convert mbox_mbox_sync() to use buffer pool
 * Change ssl_init to use buffer pool for rand filename
 * Convert mutt_save_confirm() to use buffer pool
 * Convert mx_mbox_close() to use buffer pool
 * Convert pgp_app_handler to use buffer pool
 * Convert pgp_check_traditional_one_body() to use buffer pool
 * Convert pgp_class_verify_one() to use buffer pool
 * Convert pgp_extract_keys_from_attachment() to use buffer pool
 * Convert pgp_decrypt_part() to use buffer pool
 * Convert pgp_class_sign_message to use buffer pool
 * Convert pgp_class_encrypt_message() to use buffer pool
 * Convert pgp_class_traditional_encryptsign() to use buffer pool
 * Convert pgp_select_key() to use buffer pool
 * Convert pgp_class_make_key_attachment() to use buffer pool
 * Convert pop_fetch_message() to use buffer pool
 * Convert mutt_num_postponed() maildir check to use buffer pool
 * Convert attach_fmt %f to use buffer pool
 * Convert attach_forward_bodies to use buffer pool
 * Convert attach_forward_msgs() to use buffer pool
 * Convert mutt_attach_reply() to use buffer pool
 * Convert mailcap_lookup() to use buffer pool for path
 * Convert send_message() to use buffer pool
 * Convert fcc to a buffer