regmatch(): remove reginput from CURLYM

regmatch(): remove reginput from CURLYM

reginput, locinput and st->locinput were being used in a little
ballet to determine the length of the first match.
This is now simply locinput - st->locinput, or its unicode equivalent;
so the code can be simplified.

Elsewhere in the block: where reginput was being used, locinput and/or
nextchr already contain the same info, so use them instead.

This is part of a campaign to eliminate the reginput variable.