Fix problems with NetworkAtomics from r23794

r22107 fixed some wrong return types for Atomics.chpl,
and r23435 and r23472 fixed that for NetworkAtomics.chpl,
but my patch r23794 inadverdently undid some of these changes.

Here I put the changes back. Some other notes:
 - I added explicit return types to read and peek methods.
   That seemed to make it fit in better but the corresponding
   change would also need to be done in Atomics.chpl for peek.

 - I did not add :void return types to e.g. add or sub methods,
   but somebody could do that if it's important to match the style
   used in Atomics.chpl

 - I fixed peek and poke for ratomicflag to work with bool instead
   of int(64).

Tested with 'make check'.