lib: replace getsock() logic with pollsets

`getsock()` calls operated on a global limit that could
not be configure beyond 16 sockets. This is no longer adequate
with the new happy eyeballing strategy.

Instead, do the following:
- make `struct easy_pollset` dynamic. Starting with
  a minimal room for two sockets, the very common case,
  allow it to grow on demand.
- replace all protocol handler getsock() calls with pollsets
  and a CURLcode to return failures
- add CURLcode return for all connection filter `adjust_pollset()`
  callbacks, since they too can now fail.
- use appropriately in multi.c and multi_ev.c
- fix unit2600 to trigger pollset growth

Closes #18164

lib/telnet.c

@@ -188,10 +188,10 @@ const struct Curl_handler Curl_handler_telnet = {
   ZERO_NULL,                            /* connect_it */
   ZERO_NULL,                            /* connecting */
   ZERO_NULL,                            /* doing */
-  ZERO_NULL,                            /* proto_getsock */
-  ZERO_NULL,                            /* doing_getsock */
-  ZERO_NULL,                            /* domore_getsock */
-  ZERO_NULL,                            /* perform_getsock */
+  ZERO_NULL,                            /* proto_pollset */
+  ZERO_NULL,                            /* doing_pollset */
+  ZERO_NULL,                            /* domore_pollset */
+  ZERO_NULL,                            /* perform_pollset */
   ZERO_NULL,                            /* disconnect */
   ZERO_NULL,                            /* write_resp */
   ZERO_NULL,                            /* write_resp_hd */