Add most useful relaxed and acquire-relase stdatomic convenience macros

Add most useful relaxed and acquire-relase stdatomic convenience macros

The header file <isc/atomic.h> now contains convenience macros for
most useful explicit memory ordering for C11 stdatomics, only relaxed
and acquire-release semantics is being used.  These macros SHOULD be
used instead of atomic_<func>_explicit functions.