regcomp.h: Add better named synonyms

regcomp.h: Add better named synonyms

This continues the process started two commits ago of removing some of
the overloading of the term 'class'.

In this case, this commit adds some #defines referring to the portions
of the regnode associated with bracketed character classes, the ANYOF
node.  Specifically those portions that deal with the Posix character
classes, like \w and [:punct:] under /l (locale) matching are renamed
substituting POSIXL for CLASS.  POSIXL is already used for POSIX-related
things under /l.  I remember being terribly confused when I started
reading this code about this.  One had a class within a class.  This
should clarify things somewhat.

The old names are retained in case files outside the core #include and
use it (there are a few such in cpan).