utf8.h: Add clearer #define synonyms

utf8.h: Add clearer #define synonyms

These names have long caused me consternation, as they are named after
the internal ASCII-platform UTF-8 representation, which is not the same
for EBCDIC platforms, nor do they convey meaning to someone who isn't
currently steeped in the UTF-8 internals.  I've added synonyms that are
platform-independent in meaning and make more sense to someone coming at
this cold.  The old names are retained for back compat.