MINOR: mworker: allow pidfile in mworker + foreground

This patch allows the use of the pidfile in master-worker mode without
using the background option.
diff --git a/src/haproxy.c b/src/haproxy.c
index bbd26b8..f12e903 100644
--- a/src/haproxy.c
+++ b/src/haproxy.c
@@ -2499,7 +2499,7 @@
 	}
 
 	/* open log & pid files before the chroot */
-	if (global.mode & MODE_DAEMON && global.pidfile != NULL) {
+	if ((global.mode & MODE_DAEMON || global.mode & MODE_MWORKER) && global.pidfile != NULL) {
 		unlink(global.pidfile);
 		pidfd = open(global.pidfile, O_CREAT | O_WRONLY | O_TRUNC, 0644);
 		if (pidfd < 0) {