posix_openpt: Support for OpenBSD.
* lib/posix_openpt.c [OpenBSD]: Include <sys/ioctl.h>, <sys/tty.h>.
(posix_openpt) [OpenBSD]: New code.
* lib/grantpt.c: Include <fcntl.h>.
(grantpt) [OpenBSD]: Only test whether fd is valid, nothing else.
* modules/grantpt (Depends-on): Add fcntl-h.