ptmx, /dev/pts support for 5.x

Last week, I did a bit of experimentation to look at privilege issues in
the allocation of ptys on FreeBSD.  Right now, to safely allocate a
pty/tty pair, you basically need privilege (otherwise you can't
chown/chmod the slave node).  I took a look at some of the approaches used
by other systems (specifically Linux, Solaris) and implemented basic ptmx
support for FreeBSD:

  http://www.watson.org/~robert/freebsd/pts/

I took the existing tty_pty.c driver code and remunged most of the dev_t
and clone handling, as well as open/close of both master and slave.  It's
far from perfect -- more of an experiment really, but it has some notable
benefits, including allowing programs like window, xterm, etc, to allocate
ptys safely without privilege (although utmp still requires privilege if
you're into that sort of thing).  pts/ttys are also GC'd to reset
ownership/permissions/flags in a race-free manner. 

The one bug I've been bumping my head against and haven't yet manage to
resolve is that controlling tty support appears to be broken -- perhaps a
property of the ordering of dev_t vs. tty initialization.

The one other caveat is that to allocate a new pty, it's sufficient to
lookup /dev/ptmx, not just open it, due to our devfs cloning approach.  I
probably need some timeout to talk along and GC allocated but unopened
ptys.

I include some lightweight implementations of openpty(), grantpt(), etc,
based on the kernel code.  Ideally, the libraries would support backward
compatibility, however, preferring ptmx/pts over pty/tty but supporting
kernels with either...

Thoughts, etc, welcome -- I probably don't have time to follow through
effectively on this for a few weeks, so if someone else wants to grab and
run with it (perhaps throwing out my implementation on the way), they
should feel free.

Robert N M Watson             FreeBSD Core Team, TrustedBSD Projects
robert_at_fledge.watson.org      Network Associates Laboratories