Add hook for Unicode private use override

Add hook for Unicode private use override

I am starting to write a Unicode::Private_Use module which will allow
one to specify the Unicode properties of private use code points, thus
making them actually useful.  This commit adds a hook to regcomp.c to
accommodate this module.  The changes are pretty minimal.  This way we
don't have to wait another release cycle to get it out there.

I don't want to document this interface, until it's proven.