MFC r259634:

MFC r259634:
  Prevent users from deactivating the last component of a mirror.

MFC r259929:
  Add an ability to stop gmirror and clear its metadata in one command.
  This fixes the problem, when gmirror starts again just after stop.

  The problem occurs when gmirror's component has geom label with equal size.
  E.g. gpt and gptid have the same size as partition, diskid has the same
  size as entire disk. When gmirror's geom has been destroyed, glabel
  creates its providers and this initiate retaste.

  Now "gmirror destroy" command is available. It destroys geom and also
  erases gmirror's metadata.

PR:		184985

git-svn-id: svn://svn.freebsd.org/base/stable/10@260503 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f