Don't let SDL catch SIGINT and SIGTERM signals. This is not suitable for

SheepShaver since we are typically translating SDL_QUIT events to PowerOff()
on MacOS side. And, if MacOS is not fully booted, it's not really convenient
to shut it down, even with ctrl-C. i.e. you had to kill -9 it.

SheepShaver/src/Unix/main_unix.cpp

@@ -467,6 +467,10 @@ int main(int argc, char **argv)
 		goto quit;
 	}
 	atexit(SDL_Quit);
+
+	// Don't let SDL catch SIGINT and SIGTERM signals
+	signal(SIGINT, SIG_DFL);
+	signal(SIGTERM, SIG_DFL);
 #endif
 
 #ifndef USE_SDL_VIDEO