Simple lock-free singly-linked stack

A singly-linked list that supports lock-free prepend and drain (to empty the list and clean up its elements). Intended for use with QSBR to collect objects that need safe memory reclamation.

In <isc/atomic.h>, add an atomic_ptr() macro to make type declarations a little less abominable, and clean up a duplicate definition of atomic_compare_exchange_strong_acq_rel()