regexec.c: Handle REFFU and NREFFU; refactor

regexec.c: Handle REFFU and NREFFU; refactor

This adds handling of the Unicode folding semantics capture buffer
backreferences.  I've refactored the code so that the case statements
set up the type of folding, to avoid having to test for which in the
common code.

Also, the previous code was confusing fold case and lowercase.  There is
already a routine to handle the fold case, so that simplified things.