freebsd: add utmpx constants
freebsd: add utmpx constants

This adds a few missing constants from FreeBSD `utmpx.h`.

Ref: https://github.com/freebsd/freebsd/blob/a1d2b5187332a366a897689c5fb41d38d7e1b0d8/include/utmpx.h

rust-lang/libc,rust-lang/libc,rust-lang/libc