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/smb.c

@@ -329,7 +329,7 @@ const struct Curl_handler Curl_handler_smb = {
   PORT_SMB,                             /* defport */
   CURLPROTO_SMB,                        /* protocol */
   CURLPROTO_SMB,                        /* family */
-  PROTOPT_NONE                          /* flags */
+  PROTOPT_CONN_REUSE                    /* flags */
 };
 
 #ifdef USE_SSL
@@ -358,7 +358,7 @@ const struct Curl_handler Curl_handler_smbs = {
   PORT_SMBS,                            /* defport */
   CURLPROTO_SMBS,                       /* protocol */
   CURLPROTO_SMB,                        /* family */
-  PROTOPT_SSL                           /* flags */
+  PROTOPT_SSL | PROTOPT_CONN_REUSE      /* flags */
 };
 #endif
 
@@ -513,9 +513,6 @@ static CURLcode smb_connect(struct Curl_easy *data, bool *done)
   if(!smbc->send_buf)
     return CURLE_OUT_OF_MEMORY;
 
-  /* Multiple requests are allowed with this connection */
-  connkeep(conn, "SMB default");
-
   /* Parse the username, domain, and password */
   slash = strchr(conn->user, '/');
   if(!slash)