add taskqueue_drain_all

This API has semantics similar to that of taskqueue_drain but acts on
all tasks that might be queued or running on a taskqueue.
A caller must ensure that no new tasks are being enqueued otherwise this
call would be totally meaningless.  For example, if the tasks are
enqueued by an interrupt filter then its interrupt must be disabled.

MFC after:	10 days
Add some text to clarify that nslookup and dig are no longer in the base system
after the removal of BIND.
opensolaris compat: add taskq_wait emulation

MFC after:	10 days
 - Add bucket size column to `show uma` DDB command.
 - Add `show umacache` command to show alike stats for cache-only UMA zones.