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/cf-https-connect.c

@@ -35,6 +35,7 @@
 #include "multiif.h"
 #include "cf-https-connect.h"
 #include "http2.h"
+#include "select.h"
 #include "vquic/vquic.h"
 
 /* The last 3 #include files should be in this order */
@@ -426,22 +427,24 @@ static CURLcode cf_hc_shutdown(struct Curl_cfilter *cf,
   return result;
 }
 
-static void cf_hc_adjust_pollset(struct Curl_cfilter *cf,
-                                 struct Curl_easy *data,
-                                 struct easy_pollset *ps)
+static CURLcode cf_hc_adjust_pollset(struct Curl_cfilter *cf,
+                                     struct Curl_easy *data,
+                                     struct easy_pollset *ps)
 {
+  CURLcode result = CURLE_OK;
   if(!cf->connected) {
     struct cf_hc_ctx *ctx = cf->ctx;
     size_t i;
 
-    for(i = 0; i < ctx->baller_count; i++) {
+    for(i = 0; (i < ctx->baller_count) && !result; i++) {
       struct cf_hc_baller *b = &ctx->ballers[i];
       if(!cf_hc_baller_is_active(b))
         continue;
-      Curl_conn_cf_adjust_pollset(b->cf, data, ps);
+      result = Curl_conn_cf_adjust_pollset(b->cf, data, ps);
     }
-    CURL_TRC_CF(data, cf, "adjust_pollset -> %d socks", ps->num);
+    CURL_TRC_CF(data, cf, "adjust_pollset -> %d, %d socks", result, ps->n);
   }
+  return result;
 }
 
 static bool cf_hc_data_pending(struct Curl_cfilter *cf,