BUG/MINOR: mworker: disable busy polling in the master process

When enabling busy polling, we don't want the master to use it, or it
wastes a dedicated processor to this!

Must be backported to 1.9.
diff --git a/src/haproxy.c b/src/haproxy.c
index 3477ca5..fb6481b 100644
--- a/src/haproxy.c
+++ b/src/haproxy.c
@@ -650,6 +650,8 @@
 	if (global.tune.options & GTUNE_USE_SYSTEMD)
 		sd_notifyf(0, "READY=1\nMAINPID=%lu", (unsigned long)getpid());
 #endif
+	/* Busy polling makes no sense in the master :-) */
+	global.tune.options &= ~GTUNE_BUSY_POLLING;
 
 	master = 1;