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.cxx

@@ -20,6 +20,8 @@
 #include "config.h"
 #include "Thread.hxx"
 #include "thread/Name.hxx"
+#include "thread/Util.hxx"
+#include "Log.hxx"
 
 void
 EventThread::Start()
@@ -41,7 +43,16 @@ EventThread::Stop() noexcept
 void
 EventThread::Run() noexcept
 {
-	SetThreadName("io");
+	SetThreadName(realtime ? "rtio" : "io");
+
+	if (realtime) {
+		try {
+			SetThreadRealtime();
+		} catch (...) {
+			LogError(std::current_exception(),
+				 "RTIOThread could not get realtime scheduling, continuing anyway");
+		}
+	}
 
 	event_loop.Run();
 }