utf8.h, utfebcdic.h: Create less confusing #defines

utf8.h, utfebcdic.h: Create less confusing #defines

This commit creates macros whose names mean something to me, and which I
don't find confusing.  The older names are retained for backwards
compatibility.  Future commits will fix bugs I introduced from
misunderstanding the meaning of the older names.

The older names are now #defined in terms of the newer ones, and moved
so that they are only defined once, valid for both ASCII and EBCDIC
platforms.