- Add real assertions to lockmgr locking primitives.

  A couple of notes for this:
  * WITNESS support, when enabled, is only used for shared locks in order
    to avoid problems with the "disowned" locks
  * KA_HELD and KA_UNHELD only exists in the lockmgr namespace in order
    to assert for a generic thread (not curthread) owning or not the
    lock.  Really, this kind of check is bogus but it seems very
    widespread in the consumers code.  So, for the moment, we cater this
    untrusted behaviour, until the consumers are not fixed and the
    options could be removed (hopefully during 8.0-CURRENT lifecycle)
  * Implementing KA_HELD and KA_UNHELD (not surported natively by
    WITNESS) made necessary the introduction of LA_MASKASSERT which
    specifies the range for default lock assertion flags
  * About other aspects, lockmgr_assert() follows exactly what other
    locking primitives offer about this operation.

- Build real assertions for buffer cache locks on the top of
  lockmgr_assert().  They can be used with the BUF_ASSERT_*(bp)
  paradigm.

- Add checks at lock destruction time and use a cookie for verifying
  lock integrity at any operation.

- Redefine BUF_LOCKFREE() in order to not use a direct assert but
  let it rely on the aforementioned destruction time check.

KPI results evidently broken, so __FreeBSD_version bumping and
manpage update result necessary and will be committed soon.

Side note: lockmgr_assert() will be used soon in order to implement
real assertions in the vnode namespace replacing the legacy and still
bogus "VOP_ISLOCKED()" way.

Tested by:      kris (earlier version)
Reviewed by:    jhb

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