regcomp.h: Create new typedef synonym for clarity

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.