[striconveh] Error handling and Unicode replacement character

[Marc Nieper-Wißkirchen]

The striconveh module and related modules offer an error handler
argument. The current possible values are:

iconveh_error
iconveh_question_mark
iconveh_escape_sequence

The second option replaces any unconvertible character with a question mark "?".

I would like to request to add a fourth option, say,
iconveh_replacement_character, which is like iconveh_question_mark but
uses U+FFFD whenever the target codeset is a Unicode codeset.

Thanks,

Marc