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.
WIP: Make dump.c nul-and-UTF8 clean