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.
MFC after: 2 weeks