Allow allowing UTF-8 overflow malformation

Allow allowing UTF-8 overflow malformation

perl has never allowed the UTF-8 overflow malformation, for some reason.
But as long as overflows are turned into the REPLACEMENT CHARACTER,
there is no real reason not to.  And making it allowable allows code
that wants to carry on in the face of malformed input to do so, without
risk of contaminating things, as the REPLACEMENT is the Unicode
prescribed way of handling malformations.