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

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

Rename the generic "CLASS" to the more specific "GEOM_CLASS".
While I'm here remove redundancy and inconsistencies.

Obtained from: Juniper Networks

Fixed dependencies (make checkdpadd).

- Initialize variable in order to avoid GCC warning and enable WARNS=6.

PR: bin/139970
Submitted by: Ulrich Spörlein <uqs -at- spoerlein.net>

Use humanize_number(), rather than a home-grown algorithm for
formatting a number in a human-friendly way.

Note that with this commit a megabyte changed from 1000000 to
1048576 and a 80G disk is now printed as being 75G in size.
This is deliberate. It's consistent with the core of geom(8).
However, the original choice for a megabyte being 1000000 was
on purpose and matches what disk vendors put on the box. The
consistency is considered more important.

Submitted by: delphij

Add gpart(8).

In order to support gpart(8), geom(8) needs to support a named
argument. Also, optional string parameters are a requirement.
Both have been added to the infrastructure. The former required
all existing classes to be adjusted.