Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

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

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

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

The default balance algorithm has changed from "split" to
(the improved version of) "load".

Actually component with the greatest priority is used by the prefer balance
algorithm.

Add support for changing providers priority.

Submitted by: Mel Flynn

Remove duplicate "clear" subcommand.

PR: docs/106947
Submitted by: TAOKA Fumiyoshi

Bump .Dd for -f|-F.

Now, that we have gjournal in the tree add possibility to configure
gmirror and graid3 in a way that it is not resynchronized after a
power failure or system crash.
It is safe when gjournal is running on top of gmirror/graid3.

Markup fixes.

Add some notes how to properly dump kernel onto gmirror provider.

Reviewed and corrected by: brueffer
MFC after: 3 days

gmirror.8:
Note the default balancing algorithm and stripe size.

geom_mirror.c:
Slightly friendlier error message.

Reviewed by: pjd

Add a note in example as well, that last sector is used for metadata,
so it don't provoke confusions.

Noticed by: Victor Sudakov <sudakov@sibptus.tomsk.ru>
MFC after: 2 days

Remove duplicated description of 'clear' subcommand.

Pointed out by: marck
X-MFC-after: few seconds

Document 'clear' and 'dump' subcommands.

MFC after: 1 week

Instead of documenting every standard subcommand's argument everywhere,
just leave reference to geom(8).

MFC after: 1 week

- Document 'status' subcommand.
- Update copyrights.

MFC after: 1 week

Fix a couple of grammar nits.

PR: 77437
Submitted by: Paul A. Hoadley <paulh@logicsquad.net> (original version)

Sort sections.

Added the EXIT STATUS section where appropriate.

Fix arguments syntax.

Manual pages fixes by: ru
MFC after: 3 days

Markup fixes.

Note that sysctls documentation is missing.

Grammatical and spelling improvements.

Reviewed by: pjd

Document '-p' flag for 'insert' command correct.

Add a line to BUGS section about the need of implementation description.

Add some missing empty lines.

Fix typo.

Ok, let's try again:
Add manual page for gmirror(8) utility.