MFC r306279: Use g_wither_provider() where applicable.

It is just a helper function combining G_PF_WITHER setting with
g_orphan_provider().

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

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

Add some FEATURE macros for various GEOM classes.

No FreeBSD version bump, the userland application to query the features will
be committed last and can serve as an indication of the availablility if
needed.

Sponsored by: Google Summer of Code 2010
Submitted by: kibab
Reviewed by: silence on geom@ during 2 weeks
X-MFC after: to be determined in last commit with code from this project

- Apply the same naming rules of LVM names as done in the LVM code itself.

PR: kern/135874

Add a geom class to map Linux LVM logical volumes.

The logical disks will appear as /dev/lvm/<vol group>-<logical vol>, for
instance /dev/lvm/vg0-home. G_LINUX_LVM currently supports linear stripes with
segments on multiple physical disks. The metadata is read only, logical
volumes can not be allocated or resized.

Reviewed by: Ivan Voras

Previously known as geom_lvm(4), rename requested by des, phk.