regcomp.h: Add some macros

regcomp.h: Add some macros

These are use to allow the bit map of run-time /[[:posix:]]/l classes to
be stored in a variable, and not just in the argument of an ANYOF node.
This will enable the next commit to use such a variable.  The current
macros are rewritten to just call the new ones with the proper arguments.

A macro of a different sort is also created to allow one to set the
entire bit field in the node at once.