striconveh: Support an error handler that produces a Unicode U+FFFD.
Suggested by Marc Nieper-Wißkirchen in
<https://lists.gnu.org/archive/html/bug-gnulib/2021-12/msg00175.html>.
* lib/iconveh.h (iconveh_replacement_character): New enum value.
* lib/striconveh.c (mem_cd_iconveh_internal): When the handler is
iconveh_replacement_character, try to produce U+FFFD when possible,
instead of '?'.
* tests/test-striconveh.c (main): Add GB18030 tests. Test also
iconveh_replacement_character.