Remove $FreeBSD$: two-line nroff pattern

Remove /^\.\\"\n\.\\"\s*\$FreeBSD\$$\n/

Rename libohash to libopenbsd.

libopenbsd is an internal library which
to bring in compatibility stuff from OpenBSD.
This will allow us to bring in more
OpenBSD utilities into the FreeBSD base system.

We similarly use libnetbsd for bringing in stuff from NetBSD.

Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D4078