regcomp.c: Simply some node calculations

regcomp.c: Simply some node calculations

For the node types that have differing versions depending on the
character set regex modifiers, /d, /l, /u, /a, and /aa, we can use the
enum values as offsets from the base node number to derive the correct
one.  This eliminates a number of tests.

Because there is no DIGITU node type, I added placeholders for it (and
NDIGITU) to avoid some special casing of it (more important in future
commits).  We currently have many available node types, so can afford to
waste these two.