rearrange code for add,sub,or,and,xor atomics

rearrange code for add,sub,or,and,xor atomics
- group the specialized implementations
- group the public macro definitions and supporting inline functions
mods for initial port to MacOSX (Darwin)

mcdb 'make test' passes
However, nss_mcdb has not yet been ported to use MacOSX Open Directory
(man opendirectoryd)
plasma_feature.h - OS and architecture features

preprocessor endian detection, largefile support, and more
plasma_stdtypes.h - standard types

portable standard types from sys/types.h, stdint.h, and stdbool.h
simplify code using plasma_{feature,stdtypes}.h