iconv: only conditionally use ICONV_SET_DISCARD_ILSEQ

If the -c flag is used, then we can set it with ICONV_SET_DISCARD_ILSEQ;
otherwise, leave it alone. The user may have specified //IGNORE in the
'to' codeset specification, there's no reason we can't allow that but
we'll currently turn it off.

Reviewed by: thj
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D34342