add Perl_reg_set_capture_string() function

add Perl_reg_set_capture_string() function

Cut and paste into a separate function, the block of code in
regexec_flags() that is responsible (on successful match) for setting
RX_SAVED_COPY, RX_SUBBEG etc, ready for use by capture vars like $1, $&.

Although this function is currently only called from one place, we will
shortly use it elsewhere too.

This should contain no functional changes.