Simple lock-free stack in <isc/stack.h>

Simple lock-free stack in <isc/stack.h>

Add a singly-linked stack 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, or any other user
that works with adding objects to the stack and then draining them in
one go like various work queues.

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()`