Prevent multiple ntpds from tripping over each other.

This brings over the logic from bgpd & ospfd.
Input & OK deraadt

src/usr.sbin/ntpd/control.c

@@ -1,4 +1,4 @@
-/*	$OpenBSD: control.c,v 1.13 2018/08/04 11:07:14 mestre Exp $ */
+/*	$OpenBSD: control.c,v 1.14 2019/01/14 16:30:21 florian Exp $ */
 
 /*
  * Copyright (c) 2003, 2004 Henning Brauer <henning@openbsd.org>
@@ -36,6 +36,32 @@
 
 #define square(x) ((x) * (x))
 
+int
+control_check(char *path)
+{
+	struct sockaddr_un	 sun;
+	int			 fd;
+
+	bzero(&sun, sizeof(sun));
+	sun.sun_family = AF_UNIX;
+	strlcpy(sun.sun_path, path, sizeof(sun.sun_path));
+
+	if ((fd = socket(AF_UNIX, SOCK_STREAM, 0)) == -1) {
+		log_warn("control_check: socket check");
+		return (-1);
+	}
+
+	if (connect(fd, (struct sockaddr *)&sun, sizeof(sun)) == 0) {
+		log_warnx("control_check: socket in use");
+		close(fd);
+		return (-1);
+	}
+
+	close(fd);
+
+	return (0);
+}
+
 int
 control_init(char *path)
 {

src/usr.sbin/ntpd/ntpd.c

@@ -1,4 +1,4 @@
-/*	$OpenBSD: ntpd.c,v 1.119 2018/11/29 14:25:07 tedu Exp $ */
+/*	$OpenBSD: ntpd.c,v 1.120 2019/01/14 16:30:21 florian Exp $ */
 
 /*
  * Copyright (c) 2003, 2004 Henning Brauer <henning@openbsd.org>
@@ -202,6 +202,9 @@ main(int argc, char *argv[])
 			    pname);
 
 		fatalx("%s: process '%s' failed", __func__, pname);
+	} else {
+		if ((control_check(CTLSOCKET)) == -1)
+			fatalx("ntpd already running");
 	}
 
 	if (setpriority(PRIO_PROCESS, 0, -20) == -1)

src/usr.sbin/ntpd/ntpd.h

@@ -1,4 +1,4 @@
-/*	$OpenBSD: ntpd.h,v 1.137 2018/11/06 20:41:36 jsing Exp $ */
+/*	$OpenBSD: ntpd.h,v 1.138 2019/01/14 16:30:21 florian Exp $ */
 
 /*
  * Copyright (c) 2003, 2004 Henning Brauer <henning@openbsd.org>
@@ -393,6 +393,7 @@ void			sensor_query(struct ntp_sensor *);
 void			ntp_dns(struct ntpd_conf *, struct passwd *);
 
 /* control.c */
+int			 control_check(char *);
 int			 control_init(char *);
 int			 control_listen(int);
 void			 control_shutdown(int);