Remove $FreeBSD$: one-line sh pattern

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

Move the rc framework out of sbin/init into libexec/rc.

The reasons for this are forward looking to pkgbase:
* /sbin/init is a special binary; try not to replace it with
every package update because an rc script was touched.
(a follow-up commit will make init its own package)
* having rc in its own place will allow more easy replacement
of the rc framework with alternatives, such as openrc.

Discussed with: brd (during BSDCam), kmoore
Requested by: cem, bz
PR: 231522
Approved by: re (gjb)