libc: Purge unneeded cdefs.h

These sys/cdefs.h are not needed. Purge them. They are mostly left-over
from the $FreeBSD$ removal. A few in libc are still required for macros
that cdefs.h defines. Keep those.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D42385

libc: iconv: zero out cv_shared on allocation

Right now we have to zero-initialize most fields in the varius callers,
but this is a little error prone. Simplify it by zeroing it out upon
allocation instead, drop the other redundant initialization.

Reviewed by: markj
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D41546

libc: fix c*rtomb/mbrtoc*

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.

This restores the exact flow in iconv_std to precisely how it happened
prior to 693f88c9da8d.

PR: 265871
Fixes: 693f88c9da8d ("iconv_std: complete the //IGNORE support")
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D41513

Remove $FreeBSD$: one-line .c pattern

Remove /^[\s*]*__FBSDID\("\$FreeBSD\$"\);?\s*\n/

spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD

The SPDX folks have obsoleted the BSD-2-Clause-FreeBSD identifier. Catch
up to that fact and revert to their recommended match of BSD-2-Clause.

Discussed with: pfg
MFC After: 3 days
Sponsored by: Netflix

Reset persistent mbstates when rune locale encoding changes.

This was shown to be a problem by side effect of now-enabled test case,
which was going through C, en_US.UTF-8, ja_JP.SJIS, and ja_JP.eucJP,
and failing eventually as data in mbrtowc's mbstate, that was
perfectly correct for en_US.UTF-8 was treated as incorrect for
ja_JP.SJIS, failing the entire test case.

This makes the persistent mbstates to be per ctype-component,
and not per-locale so we could easily reset the mbstates when
only LC_CTYPE is changed.

Reviewed by: bapt, pfg
Approved by: kib (mentor, implicit)
Differential Revision: https://reviews.freebsd.org/D17796

libc: further adoption of SPDX licensing ID tags.

Mainly focus on files that use BSD 2-Clause license, however the tool I
was using mis-identified many licenses so this was mostly a manual - error
prone - task.

The Software Package Data Exchange (SPDX) group provides a specification
to make it easier for automated tools to detect and summarize well known
opensource licenses. We are gradually adopting the specification, noting
that the tags are considered only advisory and do not, in any way,
superceed or replace the license texts.

Remove the const qualifier from iconv(3) to comply with POSIX:
http://pubs.opengroup.org/onlinepubs/9699919799/functions/iconv.html

Adjust all code that calls iconv.

PR: 199099
Exp-run by: antoine
MFC after: 2 weeks

Replace the #define for "iconv" so it is for the function name instead of
a macro with parameters. Remove a __DECONST hack and add consts instead
for gnu libiconv API compatability. This makes it work with things like
devel/boost-libs that expects to use "iconv" as though it were a pointer.

Add libiconv based versions of *c16*() and *c32*().

I initially thought wchar_t was locale independent, but this seems to be
only the case on Linux. This means that we cannot depend on the *wc*()
routines to implement *c16*() and *c32*(). Instead, use the Citrus
libiconv that is part of libc.

I'll see if there is anything I can do to make the existing functions
somewhat useful in case the system is built without libiconv in the
nearby future. If not, I'll simply remove the broken implementations.

Reviewed by: jilles, gabor