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

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

MFC r238202:
Remove ancient vnconfig symlink

Approved by: cperciva (implicit)

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

Approved by: re (implicit)

Make mdconfig(8) WARNS=6 clean:

- Constify geom_config_get() name argument.
- Add void keyword for usage().
- Initialize mdunit to NULL.
- Don't call md_prthumanval() at all if length is NULL.

Approved by: trasz (mentor)

Switch the default WARNS level for sbin/ to 6.

Submitted by: Ulrich Spörlein

Make WITHOUT_DYNAMICROOT=foo work again: remember to reference
libbsdxml and libsbuf.

Teach md(4) and mdconfig(8) how to understand XML. Right now there won't be
a problem with listing large number of md(4) devices. Either 'list' or
'query' mode uses XML.

Additionally, new functionality was introduced. It's possible to pass
multiple devices to -u:

# ./mdconfig -l -u md0,md1

Approved by: cognet (mentor)

- Make md(4) 64-bit clean.
After this change it should be possible to use very big md(4) devices.
- Clean up and simplify the code a bit.
- Use humanize_number(3) to print size of md(4) devices.
- Add 't' suffix which stands for terabyte.
- Make '-S' to really work with all types of devices.
- Other minor changes.

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

Reviewed by: mike

Constify mdmaybeload(), add a prototype for usage(), and get rid of a
sizeof(int) == sizeof(long) assumption; clamp down with WARNS=2.

- Backout botched attempt to introduce MANSECT feature.
- MAN[1-9] -> MAN.

Set the default manual section for sbin/ to 8.

Make md(4) and mdconfig(8) take over the role of vn(4) and vnconfig(8)
entirely as previously advertised.

md(4) adopted all assets of vn(4) some time back and has proper devfs
support and cloning abilities to boot.

Improve cmdline parsing and messages.

Add a rudimentary man-page. (I'm no mdoc wizard, feel free to attack my
mistakes and blunders)

Preliminary scaffolding for the new integrated vn+md device driver.

I decided to work on the md(4) driver and integrate the vn(4)
functionality into it mainly based on the name being more suitable.
Ideally 'vd' as in "virtual disk" would probably be the most logical
but our sound-master pointed out that this would cause uncontrollable
fits of giggles in the brits. Another complication would the needed
changes to the ramdisk boot/root functionality.

The vn driver will stay around for some time after I complete this
merge for transition reasons, and I'll make it whine to people that
they should migrate to the md(4) driver for some time before it
dies.

The kernel part of the new md(4) driver will be committed after more
testing.