Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

- Add descriptions to most of the rc scripts. Those are mostly taken from their
daemon's manpage and probably improved.
- Consistently use "filesystem" not "file system".

Approved by: bapt, brueffer
Differential Revision: D452

Prepare for the removal of set_rcvar() by changing the rcvar=
assignments to the literal values it would have returned.

The concept of set_rcvar() was nice in theory, but the forks
it creates are a drag on the startup process, which is especially
noticeable on slower systems, such as embedded ones.

During the discussion on freebsd-rc@ a preference was expressed for
using ${name}_enable instead of the literal values. However the
code portability concept doesn't really apply since there are so
many other places where the literal name has to be searched for
and replaced. Also, using the literal value is also a tiny bit
faster than dereferencing the variables, and every little bit helps.

As previously discussed, add the svn:executable property to all scripts

Use $required_modules wherever suitable. Use load_kld() in special
cases. So we get rid of quite a few lines of duplicated code.

Use more rc.subr(8) features.
Skip useless work when ${natd_interface} is unset.
Double quote user-supplied vars unless there is a reason not to.

Automatically load the ipdivert module if it was not compiled into the kernel
and natd_enable is true.

Remove the requirement for the FreeBSD keyword as it no longer
makes any sense.

Discussed with: dougb, brooks
MFC after: 3 days

Add separate script for natd. This fixes race condition with "ipfw restart"
(when new natd is started before old natd died) and allows to manage natd
without touching ipfw.

natd should probably be killed with SIGKILL when stopping natd.