MFC r290548:

Userspace part of reroot support. This makes it possible to change
the root filesystem without full reboot, using "reboot -r". This can
be used to to eg. boot from a temporary md_image preloaded by loader(8),
setup an iSCSI session, and continue booting from rootfs mounted over
iSCSI.

Relnotes: yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D3693

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

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

Make init(8) slightly more robust when /dev/console is missing.

If the environment doesn't offer a working /dev/console, the existing
version of init(8) will simply refuse running rc(8) scripts. This means
you'll only have a system running init(8) and nothing else.

Change the code to do the following:

- Open /dev/console like we used to do, but make it more robust to use
O_NONBLOCK to prevent blocking on a carrier.
- If this fails, use /dev/null as stdin and /var/log/init.log as stdout
and stderr.
- If even this fails, use /dev/null as stdin, stdout and stderr.

So why us this useful? Well, if you remove the `getpid() == 1' check in
main(), you can now use init(8) inside jails to properly execute rc(8).
It still requires some polishing, as existing tools assume init(8) has
PID 1.

Also it is now possible to use use init(8) on `headless' devices that
don't even have a serial boot console.

Remove advertising clause from University of California Regent's license,
per letter dated July 22, 1999.

Approved by: core, imp

Add /etc/rc.shutdown capability to init.
Add sample /etc/rc.shutdown (which is just a shell for now).
Submitted by: Ollivier Robert <roberto@keltia.freenix.fr>

This commit was generated by cvs2svn to compensate for changes in r1558,
which included commits to RCS files with non-trunk default branches.

BSD 4.4 Lite sbin Sources

Note: XNSrouted and routed NOT imported here, they shall be imported with
usr.sbin.