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/event/Thread.hxx

@@ -32,9 +32,12 @@ class EventThread final {
 
 	Thread thread;
 
+	const bool realtime;
+
 public:
-	EventThread()
-		:event_loop(ThreadId::Null()), thread(BIND_THIS_METHOD(Run)) {}
+	explicit EventThread(bool _realtime=false)
+		:event_loop(ThreadId::Null()), thread(BIND_THIS_METHOD(Run)),
+		 realtime(_realtime) {}
 
 	~EventThread() noexcept {
 		Stop();