Server exit status 0 on SIGTERM/SIGINT

The previous exit status 1 would cause supervisor programs such as
systemd to report failure status on a "systemctl stop dropbear". Given
it's a graceful cleanup a status of 0 is appropriate. There doesn't seem
to be any other convention for "exited in response to SIGTERM".

src/svr-main.c

@@ -226,7 +226,7 @@ static void main_noinetd(int argc, char ** argv, const char* multipath) {
 
 		if (ses.exitflag) {
 			unlink(svr_opts.pidfile);
-			dropbear_exit("Terminated by signal");
+			dropbear_close("Terminated by signal");
 		}
 
 		if (val == 0) {