iconv_std: complete the //IGNORE support

Previously, it would only ignore failures due to csmapper conversion
failure. It may be the case that the input string contains invalid
sequences that also need to be ignored.

A good example of //IGNORE application is sanitizing user- or remotely-
specified strings that are expected to be UTF-8; perhaps as part of a
pipeline that will feed the result into a system less tested against or
tolerant of illegal UTF-8 sequences.

Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D34345