For internal use, we expect psrefs to be held by other LWPs when
asking in the cross-call thread whether the target in question is
referenced.
For public use, the caller should not assert about some other LWP
holding a psref.
But the rest of the logic is the same.
Man page says <sys/param.h>, but that's wrong.
The real problem was that rump didn't have splraiseipl at all, and
that already got fixed before I woke up this morning and panicked
because I had obviously broken all the builds!
Discussed on tech-kern:
https://mail-index.netbsd.org/tech-kern/2016/01/24/msg020069.html
API is still experimental and likely to change. (Obvious changes:
either remove extra arguments everywhere, or shrink psref_target to a
single bit, at the expense of possibly valuable diagnostic checks.)
Should do some real testing before we use this in anger!
