multi: improve wakeup and wait code

- Split WINSOCK and POSIX code in `multi_wait()` as the ifdef'ery
  was becoming unreadable
- define `ENABLE_WAKEUP` to mean the wakeup socketpair is enabled,
  no additional USE_WINSOCK check needed. Under WINSOCK
  `ENABLE_WAKEUP` is not defined, so it's availability is as before
  under the double defined() checks
- When the multi handle has "alive" transfers, the admin handle's
  pollset include the wakeup receive socket. This results in the
  admin handle running when someone uses `curl_multi_wakeup()`.
- Without any "alive" transfers, the wakeup socket is removed from
  the pollset. Otherwise, event based processing would never finish,
  eg. leave the event loop.
- The wakeup socket was never registered for event processing before,
  e.g. `curl_multi_wakeup()` never worked in that mode.
- Adjust test exepectations on socket callback invocations and
  number of sockets appearing in waitfds sets.

Closes #20832

lib/multihandle.h

@@ -71,7 +71,7 @@ typedef enum {
 
 #define CURLPIPE_ANY (CURLPIPE_MULTIPLEX)
 
-#ifndef CURL_DISABLE_SOCKETPAIR
+#if !defined(CURL_DISABLE_SOCKETPAIR) && !defined(USE_WINSOCK)
 #define ENABLE_WAKEUP
 #endif
 
@@ -160,12 +160,11 @@ struct Curl_multi {
 
 #ifdef USE_WINSOCK
   WSAEVENT wsa_event; /* Winsock event used for waits */
-#else
+#endif
 #ifdef ENABLE_WAKEUP
   curl_socket_t wakeup_pair[2]; /* eventfd()/pipe()/socketpair() used for
                                    wakeup 0 is used for read, 1 is used
                                    for write */
-#endif
 #endif
   unsigned int max_concurrent_streams;
   unsigned int maxconnects; /* if >0, a fixed limit of the maximum number of