Added missing atomic_sub_64_nv(), fixed atomic_dec_64_nv().

Existing atomic_dec_64_nv() was actually atomic_sub_64_nv(), called with missing 2nd argument.
Compiling with -Werror probably would have catched this earlier.
Fixed atomic prototypes: These functions take unsigned values, see illumos uts/common/sys/atomic.h
Cleaned up forgotten OSAtomicXXX class.  This continues commit d7b0efae.
Panic on suspective rw-locks.