term: send SIGHUP before SIGTERM when shutting down

If we're the ones initiating shutdown, start by sending SIGHUP. Only
if the client application does not terminate, send SIGTERM (and if it
still refuses to terminate, send SIGKILL).

Also reduce the timeout between the signals from 60s to 30s.

terminal.h

@@ -723,6 +723,7 @@ struct terminal {
         bool client_has_terminated;
         int terminate_timeout_fd;
         int exit_status;
+        int next_signal;
 
         void (*cb)(void *data, int exit_code);
         void *cb_data;