Modernize Thread

- Based on C++11's `thread` and `thread_local`
- No more need to allocate-deallocate or check for null
- No pointer anymore, just a member variable
- Platform-specific implementations no longer needed (except for the few cases of non-portable functions)
- Simpler for `NO_THREADS`
- Thread ids are now the same across platforms (main is 1; others follow)

platform/linuxbsd/joypad_linux.cpp

@@ -73,13 +73,12 @@ void JoypadLinux::Joypad::reset() {
 JoypadLinux::JoypadLinux(Input *in) {
 	exit_udev = false;
 	input = in;
-	joy_thread = Thread::create(joy_thread_func, this);
+	joy_thread.start(joy_thread_func, this);
 }
 
 JoypadLinux::~JoypadLinux() {
 	exit_udev = true;
-	Thread::wait_to_finish(joy_thread);
-	memdelete(joy_thread);
+	joy_thread.wait_to_finish();
 	close_joypad();
 }