Eliminate 'value shift followed by expansion' false code defect warning

Eliminate 'value shift followed by expansion' false code defect warning

* src/atomic_ops.c [AO_USE_NANOSLEEP] (AO_pause): Use "L" suffix for
shifted immediate value (1) to match the type of ts.tv_nsec.
* src/atomic_ops.c [!AO_USE_NANOSLEEP && AO_USE_WIN32_PTHREADS]
(AO_pause): Cast shifted immediate value (1) to DWORD to match Sleep
argument type.
* src/atomic_ops.c [!AO_USE_NANOSLEEP && !AO_USE_WIN32_PTHREADS]
(AO_pause): Store computed usec value to an intermediate int variable
(before storing it to tv.tv_usec) so that to avoid widening conversion
of shifted immediate int value (1); add comment.