regcomp.c: Change, variable, fcn name

regcomp.c: Change, variable, fcn name

These have always been slightly misnamed, but a recent commit made them
more so.  The old name contained "zero", but now there is a new element
which always has zero.  This element indicates whether the inversion
list is offset, that is if the beginning is the zero element, or if the
beginning is the next element beyond the zero element.