fd passing pty handling

[Kevin Steves]

We don't currently support SVR4-style fd passing which involves
I_SENDFD/I_RECVFD ioctls.  I'm not sure if that might help some
p platforms with privsep.

Some platforms that use STREAMS ptys use a set-uid root helper
to handle the privileged portions of pty creation.  grantpt(3)
has a bit more info.

on Solaris 8 it's: /usr/lib/pt_chmod
on HP-UX it's: /usr/lbin/chgpt

Perhaps this may be useful to support.  Someone want to
investigate that?