regcomp.c: Change names of some static functions

The term 'class' is very overloaded in regex code and documentation.
perlrecharclass.pod treats the dot (matching any char) as a class, and
calls the [] form "bracketed character classes".  There are other
meanings as well.  This is the first commit in a short series that
remove some of those overloadings.

One instance of class is the "synthetic start class", generated by the
regex optimizer to be a list of all the code points a sucessful match
could possibly start with.  This is useful in more quickly finding where
to start looking in matching against a target string.  Formerly, the
routines that referred to this began with 'cl_', and the formal
parameters were 'cl', which could mean any class.  This commit changes
those instances of 'cl' to 'ssc' to indicate this is the only type of
class that is being handled.
regcomp.h: Move #define

This moves it to be adjacent to similar #defines
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).
regcomp.h: Parenthesize macro formal parameter

Not doing so can cause problems, so it is standard procedure to
parenthesize all parameters within a macro definition.
regcomp.h: Create new typedef synonym for clarity

This commit finishes (at least for now) removing some of the overloading
of the term class.  A 'regnode_charclass_class' node contains space for
storing the posix classes it matches that are never defined until the
moment of matching because they are subject to the current run-time
locale.  This commit creates a typedef 'regnode_charclass_posixl'
synonym that doesn't re-use the term 'class' for two different purposes.
l
ssc f
sprout