Add -N flag to never start server even if command would normally do so,

GitHub issue 2523.

client.c

@@ -127,6 +127,8 @@ retry:
 		log_debug("connect failed: %s", strerror(errno));
 		if (errno != ECONNREFUSED && errno != ENOENT)
 			goto failed;
+		if (flags & CLIENT_NOSTARTSERVER)
+			goto failed;
 		if (~flags & CLIENT_STARTSERVER)
 			goto failed;
 		close(fd);

tmux.1

@@ -191,6 +191,11 @@ directories are missing).
 Behave as a login shell.
 This flag currently has no effect and is for compatibility with other shells
 when using tmux as a login shell.
+.It Fl N
+Do not start the server even if the command would normally do so (for example
+.Ic new-session
+or
+.Ic start-server ) .
 .It Fl S Ar socket-path
 Specify a full alternative path to the server socket.
 If

tmux.c

@@ -57,7 +57,7 @@ static __dead void
 usage(void)
 {
 	fprintf(stderr,
-	    "usage: %s [-2CDluvV] [-c shell-command] [-f file] [-L socket-name]\n"
+	    "usage: %s [-2CDlNuvV] [-c shell-command] [-f file] [-L socket-name]\n"
 	    "            [-S socket-path] [-T features] [command [flags]]\n",
 	    getprogname());
 	exit(1);
@@ -350,7 +350,7 @@ main(int argc, char **argv)
 	if (**argv == '-')
 		flags = CLIENT_LOGIN;
 
-	while ((opt = getopt(argc, argv, "2c:CDdf:lL:qS:T:uUvV")) != -1) {
+	while ((opt = getopt(argc, argv, "2c:CDdf:lL:NqS:T:uUvV")) != -1) {
 		switch (opt) {
 		case '2':
 			tty_add_features(&feat, "256", ":,");
@@ -380,6 +380,9 @@ main(int argc, char **argv)
 			free(label);
 			label = xstrdup(optarg);
 			break;
+		case 'N':
+			flags |= CLIENT_NOSTARTSERVER;
+			break;
 		case 'q':
 			break;
 		case 'S':

tmux.h

@@ -1635,7 +1635,7 @@ struct client {
 #define CLIENT_DEAD 0x200
 #define CLIENT_REDRAWBORDERS 0x400
 #define CLIENT_READONLY 0x800
-/* 0x1000 unused */
+#define CLIENT_NOSTARTSERVER 0x1000
 #define CLIENT_CONTROL 0x2000
 #define CLIENT_CONTROLCONTROL 0x4000
 #define CLIENT_FOCUSED 0x8000