As discussed on the commits list, there is no need to call revoke()
inside openpty(). On RELENG_6 and RELENG_7 unlockpt() will call
revoke(). On HEAD we create pseudo-terminals on demand, so there is no
need to revoke the slave device node.
This change should never be MFC'd, because the implementation we have in
RELENG_6 and RELENG_7 should work flawlessly with older versions of
libc.
Discussed with: jhb
MFC after: never
b8cbf1a36a