Add tcgetwinsize(3) and tcsetwinsize(3) to termios

These functions get/set tty winsize respectively, and are trivial wrappers
around corresponding termio ioctls.

The functions are expected to be a part of POSIX.1 issue 8:
https://www.austingroupbugs.net/view.php?id=1151#c3856.
They are currently available in NetBSD and in musl libc.

PR: 251868
Submitted by: Soumendra Ganguly <soumendraganguly@gmail.com>
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D27650