Add exit-empty option to exit server if no sessions (defaults to on).

options-table.c

@@ -97,6 +97,12 @@ const struct options_table_entry options_table[] = {
 	  .default_num = 500
 	},
 
+	{ .name = "exit-empty",
+	  .type = OPTIONS_TABLE_FLAG,
+	  .scope = OPTIONS_TABLE_SERVER,
+	  .default_num = 1
+	},
+
 	{ .name = "exit-unattached",
 	  .type = OPTIONS_TABLE_FLAG,
 	  .scope = OPTIONS_TABLE_SERVER,

server.c

@@ -255,6 +255,9 @@ server_loop(void)
 
 	server_client_loop();
 
+	if (!options_get_number(global_options, "exit-empty") && !server_exit)
+		return (0);
+
 	if (!options_get_number(global_options, "exit-unattached")) {
 		if (!RB_EMPTY(&sessions))
 			return (0);

tmux.1

@@ -2503,6 +2503,11 @@ Set the time in milliseconds for which
 waits after an escape is input to determine if it is part of a function or meta
 key sequences.
 The default is 500 milliseconds.
+.It Xo Ic exit-empty
+.Op Ic on | off
+.Xc
+If enabled (the default), the server will exit when there are no active
+sessions.
 .It Xo Ic exit-unattached
 .Op Ic on | off
 .Xc