libc: iconv: push option ignore into citrus_iconv_open()

Make it vaguely aware of options in the sense that it now knows that it
can zap any trailing //. It now copies the entire string in realsrc and
realdst, then terminates them at the options.

bsd_iconv_open can now stop trying to allocate memory just for this
purpose, and the new version is technically more correct. GNU libiconv
will ignore options on the in codeset and still do the right thing.

Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D34343