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/System.cpp

@@ -859,7 +859,14 @@ BootState PSP_Init(const CoreParameter &coreParam, std::string *error_string) {
 		return BootState::Failed;
 	}
 
+	return PSP_InitWait(error_string);
+}
+
+BootState PSP_InitWait(std::string *error_string) {
+	if (g_bootState == BootState::Off)
+		return BootState::Off;
 	// Every path out of the loader thread sets Complete or Failed.
+	_assert_msg_(g_loadingThread.joinable(), "bootstate: %d", (int)g_bootState.load());
 	g_loadingThread.join();
 	return PSP_InitFinish(error_string);
 }