conncontrol: reuse handling

Add protocol handler flag `PROTOPT_CONN_REUSE` to indicate that the
protocol allows reusing connections for other tranfers. Add that
to all handlers that support it.

Create connections with `conn->bits.close = FALSE` and remove all
the `connkeep()` calls in protocol handlers setup/connect implementations.
`PROTOPT_CONN_REUSE` assures that the default behaviour applies
at the end of a transfer without need to juggle the close bit.

`conn->bits.close` now serves as an additional indication that a
connection cannot be reused. Only protocol handles that allow
reuse need to set it to override the default behaviour.

Remove all `connclose()` and `connkeep()` calls from connection
filters. Filters should not modify connection flags. They are
supposed to run in eyeballing situations where a filter is just
one of many determining the outcome.

Fix http response header handling to only honour `Connection: close`
for HTTP/1.x versions.

Closes #19333

lib/rtsp.c

@@ -152,7 +152,7 @@ const struct Curl_handler Curl_handler_rtsp = {
   PORT_RTSP,                            /* defport */
   CURLPROTO_RTSP,                       /* protocol */
   CURLPROTO_RTSP,                       /* family */
-  PROTOPT_NONE                          /* flags */
+  PROTOPT_CONN_REUSE                    /* flags */
 };
 
 #define MAX_RTP_BUFFERSIZE 1000000 /* arbitrary */