Add ICONV_{GET,SET}_ILSEQ_INVALID iconvctl. GNU iconv returns EILSEQ
when there is an invalid character in the output codeset while it is
valid in the input. However, POSIX requires iconv() to perform an
implementation-defined conversion on the character. So, Citrus iconv converts
such a character to a special character which means it is invalid in the
output codeset.

This is not a problem in most cases but some software like libxml2 depends
on GNU's behavior to determine if a character is output as-is or another form
such as a character entity (&#NNN;).