OpenSystemsDevelopment / qpdf

Also fix accidental Buffer copy in Pl_LZWDecoder::addToTable.

Also, reject them in strings.

Also. remove '.' as  starting char in lsTop.

Avoid representing as PDF Doc encoding any string whose PDF Doc
encoding representation starts with a UTF-16 or UTF-8 marker.

Since Buffer has always implemented its copy constructor with a deep
copy, its Members object will never have multiple owners. Change to unique_ptr.

Also implement move constructors for Buffer, since there may be cases
where a deep copy is not needed.

When the schema wants a variable-length array, allow a single item as
well as allowing an array.