rxvt-unicode -- restore permissions on tty devices

A rxvt-unicode changelog reports:

SECURITY FIX: on systems using openpty, permissions were not correctly updated on the tty device and were left as world-readable and world-writable (likely in original rxvt, too), and were not restored properly. Affected are only systems where non-unix ptys were used (such as most BSDs). Found, patched and debugged by Ryan Beasley.

[source]