Rationalise RX_MATCH_UTF8_set()

Rationalise RX_MATCH_UTF8_set()

Now that the RXf_MATCH_UTF8 flag on a regex is just used to indicate
whether the captures on a successful match are utf8, only set
this flag at the end of a successful match, rather than at the start of
the match.

This should make no functional difference the way things stand at the
moment, but makes things conceptually cleaner.