regexec_flags(): keep stringarg constant

regexec_flags(): keep stringarg constant

stringarg is the arg passed to Perl_regexec_flags() to indicate where to
start matching. Currently the code adjusts this under \G, then copies it
to startpos, then later tinkers with startpos further.

Change it so that stringarg is never changed, and all the adjusting is to
startpos. Shouldn't make any logical difference, but makes the code
slightly cleaner and easier to understand (and doesn't require minend to
be adjusted any more).