Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

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

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

Switch the default WARNS level for sbin/ to 6.

Submitted by: Ulrich Spörlein

- Make WARNS?= 6 clean
- Add WARNS?= 6

Approved by: stefanf, grehan (mentor)

Also MLINK the man page for mount_mfs.

Forced commit to indicate that what is installed is a hard link,
not a symbolic one.

Install a mount_mfs symlink so that you can specify something like this
in /etc/fstab to get a memory disk:

md /tmp mfs rw,-s8m,noatime 2 2

Back when mdmfs was created, there was vague discussion about doing this, but
it never materialized.

Reminded by: Ralf S. Engelschall <rse@engelschall.com>

Default to WARNS=2.
Binary builds that cannot handle this must explicitly set WARNS=0.

Reviewed by: mike

Introduce mdmfs(8), a wrapper around mdconfig(8), disklabel(8),
newfs(8), and mount(8) that mimics the command line option set of the
deprecated mount_mfs(8).

Approved by: jkh, phk, -hackers