Enable posix_openpt on FreeBSD bsd-user targets.

Forward the syscall directly to the host, there's nothing special
about it. While there, enable two ttycom.h ioctls that together
with the posix_openpt implementation allow QEMU to run GNU screen
utility successfully.
Merge pull request #4 from akabaev/bsd-user

Enable posix_openpt on FreeBSD bsd-user targets.