Do reference counting for zfs range lock waiters.

Avoid cv_broadcast(&cv); cv_destroy(&cv); which works in Solaris only
by abuse of the condvar abstraction.

There are parts of this code that should be factored into smaller
subroutines, mainly range lock allocation and initialization, but
that would make it harder to merge newer versions of zfs, so for now
I've just expanded those parts further in-line.

(riastradh)