Do not start the daemon more than once

Avoid starting multiple daemon instances if the --daemon argument is
passed and the daemon is already running. Terminate the already running
instance before spawning a new one instead.

Suggested-by: Vlad Glagolev <scm@vaygr.net>
Signed-off-by: Lukas Fleischer <lfleischer@calcurse.org>

src/args.c

@@ -803,6 +803,7 @@ int parse_args(int argc, char **argv)
 		io_export_data(xfmt, export_uid);
 	} else if (daemon) {
 		notify_init_vars();
+		dmon_stop();
 		dmon_start(0);
 	} else {
 		/* interactive mode */