Remove sleeps and timed polling from networking

Socket waits now block in select() on the socket plus a wake or cancel
socket (Common/Net/Cancel), so a stop or cancel ends them at once.

- HTTP: cancel flags become net::CancelToken; Connect waits once on all
  addresses, receives wait instead of spinning, no DNS retry sleep.
- Adhoc: the friend finder and adhoc server block on their sockets with
  a wake socket and a deadline; matching runs on a CoreTiming tick
  instead of two threads (savestate section bumped).
- sceNetInet select/poll check without blocking and park the HLE thread,
  re-checked from a CoreTiming event until ready or the emulated
  timeout.
- Vendor the aemu_postoffice client (was a submodule) with no sleeps:
  waits on the socket, close via shutdown and an eventcount, cancelable
  connects, and lock-free session slot claims.
- PortManager waits without a timeout; PSP_InitWait replaces the
  headless boot poll.
- TimeUtil: sleep_us/sleep_ms only outside the core; sleep_precise,
  sleep_random, yield and the Windows timer period are gone.
- Unit test for the socket waits.

Core/WebServer.cpp

@@ -106,8 +106,7 @@ static ServerStatus RetrieveStatus() {
 static bool RegisterServer(int port) {
 	bool success = false;
 	http::Client http(nullptr);
-	bool cancelled = false;
-	net::RequestProgress progress(&cancelled);
+	net::RequestProgress progress(nullptr);
 	Buffer theVoid = Buffer::Void();
 
 	http.SetUserAgent(StringFromFormat("PPSSPP/%s", PPSSPP_GIT_VERSION));