Merge the 1 and 2 byte versions of the atomic functions into one.

After pushing in my fix for the 2 byte functions, I realized that the
functions for 1 and 2 byte operations had become identical. Reduce the
code size by merging the functions for 1 and 2 byte operations together.

While there, slightly improve variable naming and comments.
Fix a bad calloc(3) call

MFC after:	2 weeks