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 236768,252209,253047:
Several improvements to rmlock(9). Many of these are based on patches
provided by Isilon.
- Add an rm_assert() supporting various lock assertions similar to other
locking primitives. Because rmlocks track readers the assertions are
always fully accurate unlike rw_assert() and sx_assert().
- Flesh out the lock class methods for rmlocks to support sleeping via
condvars and rm_sleep() (but only while holding write locks), rmlock
details in 'show lock' in DDB, and the lc_owner method used by
dtrace.
- Add an internal destroyed cookie so that API functions can assert
that an rmlock is not destroyed.
- Make use of rm_assert() to add various assertions to the API (e.g.
to assert locks are held when an unlock routine is called).
- Give RM_SLEEPABLE locks their own lock class and always use the
rmlock's own lock_object with WITNESS.
- Various updates to the manpage.

MFC r233648:
Remove trailing whitespace per mdoc lint warning

Approved by: cperciva (implicit)

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

Approved by: re (implicit)

rmlock(9) two additions and one change/fix:
- add rm_try_rlock().
- add RM_SLEEPABLE to use sx(9) as the back-end lock in order to sleep while
holding the write lock.
- change rm_noreadtoken to a cpu bitmask to indicate which CPUs need to go
through the lock/unlock in order to synchronize. As a side effect, this
also avoids IPI to CPUs without any readers during rm_wlock.

Discussed with: ups@, rwatson@ on arch@
Sponsored by: Isilon Systems, Inc.

Make the rmlock(9) interface a bit more like the rwlock(9) interface:

- Add rm_init_flags() and accept extended options only for that variation.
- Add a flags space specifically for rm_init_flags(), rather than borrowing
the lock_init() flag space.
- Define flag RM_RECURSE to use instead of LO_RECURSABLE.
- Define flag RM_NOWITNESS to allow an rmlock to be exempt from WITNESS
checking; this wasn't possible previously as rm_init() always passed
LO_WITNESS when initializing an rmlock's struct lock.
- Add RM_SYSINIT_FLAGS().
- Rename embedded mutex in rmlocks to make it more obvious what it is.
- Update consumers.
- Update man page.

More cleanup.

Add rm_wowned(9) function to test whether the current thread owns an
exclusive lock on the passed rmlock.

Reviewed by: ups

Clean up the rmlock(9) manual page:

* Correct spelling and grammar
* Re-structure sentences in BUGS section for legibility
* Break long lines appropriately
* Break lines at full stops
* Add mdoc(9) commands where applicable
* Remove superfluous .Ft in the SYNOPSIS section
* Various other mdoc(9) fixes

Initial checkin for rmlock (read mostly lock) a multi reader single writer
lock optimized for almost exclusive reader access. (see also rmlock.9)

TODO:
Convert to per cpu variables linkerset as soon as it is available.
Optimize UP (single processor) case.