libc: fix c*rtomb/mbrtoc*

[kevans]

Summary

In 693f88c9da8d ("iconv_std: complete the IGNORE support"), we
more completely implemented IGNORE, which changed the semantics of
ci_discard_ilseq. DISCARD_ILSEQ semantics are supposed to match
//IGNORE, so we really can't do much about that particular
incompatibility. This broke c*rtomb and mbrtoc* handling of invalid
sequences, but it turns out they don't want DISCARD_ILSEQ semantics at
all; they really want the subset that we call
_CITRUS_ICONV_F_HIDE_INVALID.

While we're here, zero out cv_shared on initialization so that we don't
have to do so at every caller to make it a little less error prone.

Fixes: 693f88c9da8d ("iconv_std: complete the //IGNORE support")

[kevans]

Missed:

PR: 265871

CC @lwhsu, we should be able to un-skip the c16rtomb tests we skipped there after this.

[markj]

lib/libc/locale/cXXrtomb_iconv.h

78

Now this field is initialized to 0. Is that a bug in the change to use calloc(), or is it intentional? I don't know this code well enough to judge for myself.

[kevans]

lib/libc/locale/cXXrtomb_iconv.h

78

Ah, sorry, should have called that out specifically- yeah, the functional part of the change is setting this to false and passing the flag below in its place so that we still get EILSEQ, we just don't get the do_conv() replacement chars in iconv_std

[kevans]

Split out the zero-initialization bits, since they're not related to the fix
at hand.