utf8.h: Add synonyms for flag names

utf8.h: Add synonyms for flag names

The next commit will fix the detection of using Perl's extended UTF-8 to
be more accurate.  The current name for various flags in the API is
somewhat misleading.  What is really wanted to know is if extended UTF-8
was used, not the value of the resultant code point.

This commit basically does

    s/ABOVE_31_BIT/PERL_EXTENDED/g

It also similarly changes the name of a hash key in APItest/t/utf8.t.

This intermediary step makes the next commit easier to read.