add support for OpenBSD

OpenBSD doesn't permit direct syscalls so #260 uses the correct incantations from /x/sys/unix. Spotted while troubleshooting while step-cli errors out with most terminal operations there: https://github.com/openbsd/ports/blob/838a8869a28c415f7c55e10b022ef6422f15788b/security/step-cli/Makefile#L5-L8