First steps towards importing gvirstor into -current

Hi!

As announced, the plan was to start working on importing gvirstor into
CVS after 6.2 was successfully released. For those who don't know,
gvirstor is a GEOM class providing storage virtualisation facility, i.e.
its purpose is to offer the ability to create a virtual storage device
of arbitrarily large size (typically several terabytes) which consists
of an arbitrary number of physical storage devices (actually any
lower-level GEOM providers, including RAID devices) of arbitrary size
(typically 50 GB - 400 GB hard drives).

The latest sources are available in p4 and conveniently on FreeBSD's
wiki site: http://wiki.freebsd.org/gvirstor .

There are some known minor issues that will be fixed in the following
days (mostly related to overly verbose error messages), so (at the
suggestion of my mentor - pjd) I'm inviting interested users to try it
out and report any problems found (if there are such). Also, benchmarks
of gvirstor in close-to-real-life usage are welcome.

The idea is to stabilize the current code, and new features will be
developed later.