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

Remove $FreeBSD$: one-line .c comment pattern

Remove /^/[*/]\s*\$FreeBSD\$.*\n/

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

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.

iconvctl(3): remove superfluous NULL pointer tests

convname and dst are guaranteed to be non-NULL by iconv_open(3).
src is an array. Remove these tests for NULL pointers.
While I'm here, eliminate a strlcpy with a correct but suspicious-looking
calculation for the third parameter (i.e. not a simple sizeof).
Compare the strings in-place instead of copying.

Found by: bdrewery
Found by: Coverity
CID: 1130050, 1130056
MFC after: 3 days
Sponsored by: Dell Inc.
Differential Revision: https://reviews.freebsd.org/D6338

Fix several memory leaks, and crashes, in iconvlist(3).

- Both curitem and curitem (via the names list) was always leaked.
- malloc(3) failures lead to some leaks.
- __bsd___iconv_get_list() failure lead to a crash since its error was not
handles and __bsd___iconv_free_list() is not NULL-safe.

I have slightly refactored this to avoid extra malloc and free logic in cases
of malloc(3) failing.

There are still bad assumptions here that I did not deal with. One of which is
that the data will always have a '/' so the strchr(3) will not return NULL.

Coverity CID: 1130055 1130054 1130053

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

iconv_open: initialise ci_ilseq_invalid field of _citrus_iconv_shared
struct after allocation with malloc.

iconvlist: reduce a memory leak by copying strings only once.

Move the iconv wrapper source from libc_nonshared to libc/iconv so that
it is all in the one place again. Rename libc/iconv/iconv.c to
bsd_iconv.c. Compile the wrappers into libc.a so that WITHOUT_DYNAMICROOT
works again.

Discussed with: kib (and partly stolen from his patch)