init: Signal-safe instant shutdown

Replace the 200ms polling loop with a faster and more efficient waiting
operation.

This was tried a few times before, but given up every time because
solutions use a condition variable which is not safe for use in signals
as they need to be reentrant.

On UNIX-ish OSes, use a safe way: a pipe. When shutdown is requested
write a dummy byte to the pipe. Waiting for shutdown is a matter of a
blocking read from the pipe.

On Windows, there are no signals so using a condition variable is safe.

src/init.cpp

@@ -917,6 +917,9 @@ bool AppInitBasicSetup(const ArgsManager& args)
     // Enable heap terminate-on-corruption
     HeapSetInformation(nullptr, HeapEnableTerminationOnCorruption, nullptr, 0);
 #endif
+    if (!InitShutdownState()) {
+        return InitError(Untranslated("Initializing wait-for-shutdown state failed."));
+    }
 
     if (!SetupNetworking()) {
         return InitError(Untranslated("Initializing networking failed."));