Prepare for Unicode 6.2

Prepare for Unicode 6.2

This changes code to be able to handle Unicode 6.2, while continuing to
handle all prevrious releases.

The major change was a new definition of \X, which adds a property to
its calculation.  Unfortunately \X is hard-coded into regexec.c, and so
has to revised whenever there is a change of this magnitude in Unicode,
which fortunately isn't all that often.  I refactored the code in
mktables to make it easier next time there is a change like this one.