pad.c, S_cv_clone: Maintain the utf8-ness of the cloned cv

Because of a missing SvUTF8_on() in cv_clone(), these two were different:

    use utf8;
    eval "   sub foo ($;\x{30cd});"
    eval "my sub foo ($;\x{30cd});"

Because the lexical version would lose the UTF8 flag in the
prototype.
make sure the prototype actually matches

because:

a) I think it better demonstrates the fix, the following failed without
the patch:

./perl -Ilib -le 'my $proto = "\x{30cd}"; eval "sub f($proto) {}"; print prototype(\&f); print prototype(\&f) eq $proto'

b) I can envision bugs that might preserve UTF-8 but mis-manage the content