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@207384
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