FYI: mb(9) API is finally going away

[Taylor R Campbell <riastradh%NetBSD.org@localhost>]

FYI: The mb(9) API -- consisting of the mb_read, mb_write, and
mb_memory memory barriers -- was incomplete for users (failed to cover
important use cases) and incompletely defined (not defined on some
platforms like x86).  It was intended to be removed over a decade ago
in favour of the Solaris-style membar_*; only a few MD users in-tree
remain.

I'm about to remove mb(9) altogether.  This is a step in modernizing
our memory ordering interfaces.  If you were using it for some reason,
let me know and I can help you find the appropriate replacement.