This patch adds the openpty() and forkpty() library calls to posixmodule.c,
when they are available on the target
system. (glibc-2.1-based Linux systems, FreeBSD and BSDI at least, probably
the other BSD-based systems as well.)

Lib/pty.py is also rewritten to use openpty when available, but falls
back to the old SGI method or the "manual" BSD open-a-pty
code. Openpty() is necessary to use the Unix98 ptys under Linux 2.2,
or when using non-standard tty names under (at least) BSDI, which is
why I needed it, myself ;-) forkpty() is included for symmetry.