RFC: Remove pty(4)

One of my personal goals for 11 is to get rid of cloning mechanism
entirely, and pty(4) is one of the few in-kernel drivers still relying
on such mechanism.
It's not possible, at least to my understanding, converting pty(4) to
cdevpriv(9) as happened with other drivers. This is mainly because we
always need a pair of devices (/dev/ptyXX and /dev/ttyXX) and
userspace loops over ptyXX and after it successfully opens it tries to
open the other one with the same suffix. So, having a single device is
not really enough.
My option, instead, is that of removing pty(4), which is nothing more
than a compatibility driver, and move pmtx(4) code somewhere else.
The main drawback of the removal of this is that it makes impossible
to run FreeBSD <= 7 jails and SSH into them. I personally don't
consider this a huge issue, in light of the fact that FreeBSD-7 has
been EOL for a long time, but I would like to hear other people
comments.

The code review for the proposed change can be found here:
https://reviews.freebsd.org/D659

If I won't get any objection I'll commit this in one week time, i.e.
August 27th.

-- 
Davide

"There are no solved problems; there are only problems that are more
or less solved" -- Henri Poincare