This is an MFC of 205847

This is an MFC of 205847
Change how multipath labels are created and managed. This makes it easier
to support various storage boxes which really aren't active-active.

We only write the label on the *first* provider. For all other providers
we just "add" the disk. This also allows for an "add" verb.

A usage implication is that you should specificy the currently active
storage path as the first provider.

Note that this does not add RDAC-like functionality, but better allows for
autovolumefailover configurations (additional checkins elsewhere will support
this).

git-svn-id: svn://svn.freebsd.org/base/stable/8@215932 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f