connection_check, simplified

The protocol handler method `connection_check` allowed to variable
operations to trigger with variable result bits. Only the `CONNCHECK_ISDEAD`
and `CONNRESULT_DEAD` were in use. Transform the function into
`connection_is_dead` without extra parameter and a bool result.

- Remove defines for `CONNCHECK_*` and `CONNRESULT_*`
- Rename protocol function in handler comments
- Change RTSP implementation (only protocol that uses this)

Closes #20890

lib/imap.c

@@ -2317,7 +2317,7 @@ static const struct Curl_protocol Curl_protocol_imap = {
   imap_disconnect,                  /* disconnect */
   ZERO_NULL,                        /* write_resp */
   ZERO_NULL,                        /* write_resp_hd */
-  ZERO_NULL,                        /* connection_check */
+  ZERO_NULL,                        /* connection_is_dead */
   ZERO_NULL,                        /* attach connection */
   ZERO_NULL,                        /* follow */
 };