ms-queue: cleanups, convert to C11 atomics

ms-queue: cleanups, convert to C11 atomics

This converts the "pointer" to a 64-bit type (unsigned long long, or
'pointer') divided into two 32-bit halves (ptr and count), instead of
using a union of a struct and an 'unsigned long'.

All atomics are seq_cst for now. Once the code all looks a little
better, I'll relax these to reasonable memory orderings.