Remove $FreeBSD$: one-line sh pattern

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

Add a Prometheus exporter for sysctl values.

Now that we have our sysctl tree annotated with aggregation labels,
let's go ahead and provide a very simple utility for exporting the
sysctl tree in Prometheus' format. It can either be used in conjunction
with the Prometheus node exporter or run through inetd(8).

The reason why I'm opting for having it in the base system is because it
has a pretty strong integration with some of sysctl's innards, such as
access to iterators, name lookups, metadata and type information. As I
am investigating whether we can add histograms as native types to sysctl
as well, this integration will only get stronger as we go along. That's
why it would be safer to oversee the development of this exporter
ourselves, as opposed to having it as an external project.

This exporter is remarkably compact, especially when compared to the
official Linux binary of the Prometheus node exporter (16 KB vs 12 MB).
I guess this could be an interesting aspect for monitoring embedded
FreeBSD-based systems.

Differential Revision: https://reviews.freebsd.org/D8792