Add tcgetwinsize() and tcsetwinsize() to termios.h

[Soumendra Ganguly]

Hi.
   POSIX.1 issue 8 is expected to include two new functions in
termios.h: tcgetwinsize() and tcsetwinsize(). Please see
https://www.austingroupbugs.net/view.php?id=1151#c3856. These
functions are supposed to get/set tty winsize respectively. They can
be implemented as one-line ioctl()+TIOCGWINSZ/TIOCSWINSZ calls; NetBSD
already has them.

Bug 251868 includes patches for lib/libc/gen/termios.c and
/include/termios.h that add these functions; a test program has also
been supplied. If it is appropriate to add these functions to FreeBSD
now, and if anyone is interested, then please take a took at the bug
report.

Thank you.
Soumendra