Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

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

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

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

Add "GEOM_FOX", a class which detects and selects between multiple
redundant paths to the same device.

This class reacts to a label in the first sector of the device,
which is created the following way:

# "0123456789abcdef012345..."
# "<----magic-----><-id-...>
echo "GEOM::FOX someid" | dd of=/dev/da0 conv=sync

NB: Since the fact that multiple disk devices are in fact the same
device is not known to GEOM, the geom taste/spoil process cannot
fully catch all corner cases and this module can therefore be
confused if you do the right wrong things.

NB: The disk level drivers need to do the right thing for this to
be useful, and that is not by definition currently the case.