Fix use of iconv in Boost Locale. On FreeBSD it used __ICONV_F_HIDE_INVALID
which hides invalid sequences, but what Boost really wants is that iconv
returns an error on invalid sequences like GNU libiconv does by default.
On FreeBSD ICONV_SET_ILSEQ_INVALID can be used for this. It has to be set
via iconvctl.

PR: 215393