#22 Support for posix_openpt()

[Christian Biere]

posix_openpt() is the POSIX-way to open /dev/ptmx. The
device pathname is not standardized by POSIX, so this
function should be used if available. This patch is, for
example, useful for NetBSD 3.0 and newer because aterm
doesn't have to be setuid-root then and it also doesn't
run out PTYs.

[Christian Biere]

Support for posix_openpt().