MFC: sys/geom/mirror/g_mirror.c 1.81,1.82,1.83

MFC:	sys/geom/mirror/g_mirror.c	1.81,1.82,1.83
	sys/geom/mirror/g_mirror.h	1.23
	sys/geom/mirror/g_mirror_ctl.c	1.16

Sync gmirror with HEAD:

- Introduce and use delayed-destruction functionality from a pre-sync hook,
  which means that devices will be destroyed on last close.
  This fixes destruction order problems when, eg. RAID3 array is build on
  top of RAID1 arrays.

  Requested, reviewed and tested by:	ru

- Be sure to not destroy device twice. This is only possible in theory, but
  with this change there is even no theoretical race.

- Remove dead code.
- Comment possible event miss, which isn't critical, but probably can be
  fixed by replacing the event lock usage with the queue lock.