BUG/MINOR: mworker: detach from tty when in daemon mode

This allows a calling script to show the first startup output and
know when to stop reading from stdout so haproxy can daemonize.

To be backpored to 1.8.
diff --git a/src/haproxy.c b/src/haproxy.c
index c7f21e3..e842673 100644
--- a/src/haproxy.c
+++ b/src/haproxy.c
@@ -2763,6 +2763,16 @@
 			if (global.mode & MODE_MWORKER) {
 				mworker_cleanlisteners();
 				deinit_pollers();
+
+				if ((!(global.mode & MODE_QUIET) || (global.mode & MODE_VERBOSE)) &&
+					(global.mode & MODE_DAEMON)) {
+					/* detach from the tty, this is required to properly daemonize. */
+					fclose(stdin); fclose(stdout); fclose(stderr);
+					global.mode &= ~MODE_VERBOSE;
+					global.mode |= MODE_QUIET; /* ensure that we won't say anything from now */
+					setsid();
+				}
+
 				mworker_wait();
 				/* should never get there */
 				exit(EXIT_FAILURE);