output/alsa: use a new I/O thread with real-time scheduling

The normal I/O event thread can have a large latency, e.g. when
libgnutls loads all TLS CA certificates for a https connect.  This
makes it unreliable for the ALSA I/O notifications, and causes ring
buffer xruns.  To avoid interfering with high latency events such as
CURL's, we move the ALSA I/O events to a separate I/O thread which
also obtains real-time scheduling (if possible).

Closes #221

src/Instance.cxx

@@ -40,7 +40,8 @@
 #include <exception>
 
 Instance::Instance()
-	:idle_monitor(event_loop, BIND_THIS_METHOD(OnIdle))
+	:rtio_thread(true),
+	 idle_monitor(event_loop, BIND_THIS_METHOD(OnIdle))
 {
 }