utf8.c: Was taking sizeof() wrong thing

This was a typo due to the long name.  A future commit will make it
cleaner.  The sizeof() the wrong name evaluates to the right number on
ASCII platforms, but not EBCDIC.
utf8.c: Shorten long constant names, and simplify

The previous commit fixed a typo caused by it being hard to see the
differences in a long ALL_CAP name.  This uses #defines to type the long
name only once, and compile-time variables so the expression for the
length of strings only is specified once.