regen/regcharclass.pl: Generate correct macro instead of skipping

It makes no sense to check for length safeness for The macros generated
by this program which take a single UV code point as a parameter. Prior
to this patch, it would skip trying to generate them if asked.  But,
because of the way things are structured, that means that if you need
just this and the safe versions, you can't do it so easily.  What this
commit does is generate the cp macro if requested even if the 'safe'
version of other macros are also requested.
regen/regcharclass.pl: Don't generate unused macros

Having these unused macros around just clutters up the header file
regcomp.c: Make SSC node clone safe

This just sets the ptr field in the Synthetic Start Class that will be
passed to regexec.c NULL, and clarifies the comments in regcomp.h.  See
the thread starting at http://markmail.org/message/2txwaqnjco6zodeo