rc.initdiskless: Disable soft-updates in mdmfs (again)

Re-apply the -S switch to disable soft-updates in memory disks (commit
8b1292ac5219). This might be beneficial when tmpfs(5) is not present in
the kernel, as this can cause mdmfs(8)'s auto keyword to fallback to
using md(4).

PR: 85558
MFC after: 1 week
Reviewed by: imp, emaste, brooks
Differential Revision: https://reviews.freebsd.org/D43242

rc.initdiskless: Remove bogus plus sign

Reviewed by: imp, dab
Fixes: Fixes: 93b7818226cf ("libexec/rc/rc.initdiskless: make tar's path flexible")
Differential Revision: https://reviews.freebsd.org/D43241

libexec/rc/rc.initdiskless: make tar's path flexible

Summary:
PR: 274977
Reviewed by: rgrimes,imp
Approved by: rgrimes
MFC after: 1 month

Remove $FreeBSD$: one-line sh pattern

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

rc.initdiskless: Fix a typo in a comment

- s/attemping/attempting/

MFC after:5 days

Make mdmfs verbose if diskless boot is verbose.

Approved by: ian@
Differential Revision: D17104

rc.initdiskless: fix commentary grammar after r339465

MFC after: 1 month

rc.initdiskless: add support for auxiliary NVRAM.

Currently, rc.inidiskless assumes that local system configuration
changes are kept in some mountable file system. For example,
nanobsd uses dedicated partition mounted as /cfg for this.

However, small embedded devices like MIPS routers may have no enough flash
space to keep full-blown file system but have only one or couple
small flash blocks to keep persistent local configuration overrides.

This change extends rc.initdiskless and introduces ability to run auxiliary
command /conf/T/M/extract that is supposed to extract configuration overrides
from such local storage.

For example, the command /conf/default/etc/extract may contain something like:

cd "$1" && bsdcpio --quiet -idu < /dev/map/cfg

bsdcpio command extracts compressed archive from the storage to /etc
assuming the storage is exposed by the kernel as /dev/map/cfg to userland.

PR: 204215
MFC after: 1 month

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)