Reinstate "regcomp.c: Change, variable, fcn name"

Reinstate "regcomp.c: Change, variable, fcn name"

This reverts commit 0b58015e05b2ab93b080b7c49a70bf82435363c0, which
reverted 875c4e2c5193b5245da578b222e9c93aad31d93b, thus reinstating the
latter commit.  It turns out that the error being chased down was not
due to this commit.

Its original message was (slightly revised for clarity):

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.  The renamed 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.