Remove $FreeBSD$: one-line sh pattern

Remove /^\s*#[#!]?\s*\$FreeBSD\$.*$\n/

Only set WARNS if not defined

This would allow interested parties to do experimental runs with an
environment set appropriately to raise all the warnings throughout the
build; e.g. env WARNS=6 NO_WERROR=yes buildworld.

Not currently touching the numerous instances in ^/tools.

MFC after: 1 week

Allow bootstrapping libopenbsd on Linux

The getdtablecount.c file won't compile on Linux but it seems like none of
the bootstrap tools actually need it.

Reviewed By: emaste, brooks
Differential Revision: https://reviews.freebsd.org/D14244

First pass through library packaging.

Sponsored by: The FreeBSD Foundation

Remove NO_WERROR, after fix from r292023.

Add imsg to libopenbsd.

This will help with importing OpenBSD programs such as ypldap
into the base system.

Implemtn getdtablecount() to count open file descriptors for current process.

Use underlying sysctl implemented by mjg in r290473.

PR: 194985
Reviewed by: bapt, mjg
Differential Revision: https://reviews.freebsd.org/D4084

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