regex substrs: record index of check substr

regex substrs: record index of check substr

Currently prog->substrs->data[] is a 3 element array of structures.
Elements 0 and 1 record the longest anchored and floating substrings,
while element 2 ('check'), is a copy of the longest of 0 and 1.

Record in a new field, prog->substrs->check_ix, the index of which element
was copied. (Eventually I intend to remove the copy altogether.)

Also for the anchored substr, set max_offset equal to min offset.
Previously it was left as zero and ignored, although if copied to check,
the check copy of max *was* set equal to min. Having this always set will
allow us to make the code simpler.