mro.c: Correct utf8 and bytes concatenation

mro.c: Correct utf8 and bytes concatenation

The previous commit introduced some code that concatenates a pv on to
an sv and then does SvUTF8_on on the sv if the pv was utf8.

That can’t work if the sv was in Latin-1 (or single-byte) encoding
and contained extra-ASCII characters.  Nor can it work if bytes are
appended to a utf8 sv.  Both produce mangled utf8.

There is apparently no function apart from sv_catsv that handle
this.  So I’ve modified sv_catpvn_flags to handle this if passed the
SV_CATUTF8 (concatenating a utf8 pv) or SV_CATBYTES (cancatenating a
byte pv) flag.

This avoids the overhead of creating a new sv (in fact, sv_catsv
even copies its rhs in some cases, so that would mean creating two
new svs).  It might even be worthwhile to redefine sv_catsv in terms
of this....