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

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libc: iconv: add mb_cur_min for encoder traits

A future commit will actually implement //IGNORE so that applications
using base iconv can, e.g., sanitize UTF-8 strings. To do this, the
iconv_std module needs to be able to determine the minimum width for any
given encoding so that it can skip that many bytes in the input buffer.
This is mainly an issue for UTF-16 and UTF-32.

This commit bumps shlib versions to 5 for libiconv modules to reflect
the ABI change. It also fixes OptionalObsoleteFiles to remove the
libiconv modules if WITHOUT_ICONV is in use.

re: _ENCODING_MB_CUR_MIN, note that this file (citrus_stdenc_template.h)
is included at the bottom of an encoding *implementation*, so the
implementation is free to #define it prior. UTF1632 is a good example,
as it redefines the minimum to be a property on the encodinginfo, and
the minimum is set to 2 or 4 bytes for UTF-16 and UTF-32 respectively.

Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D34344

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

As a followup to r252547, propate const down the call stack.

Add the BSD-licensed Citrus iconv to the base system with default off
setting. It can be built by setting the WITH_ICONV knob. While this
knob is unset, the library part, the binaries, the header file and
the metadata files will not be built or installed so it makes no impact
on the system if left turned off.

This work is based on the iconv implementation in NetBSD but a great
number of improvements and feature additions have been included:

- Some utilities have been added. There is a conversion table generator,
which can compare conversion tables to reference data generated by
GNU libiconv. This helps ensuring conversion compatibility.
- UTF-16 surrogate support and some endianness issues have been fixed.
- The rather chaotic Makefiles to build metadata have been refactored
and cleaned up, now it is easy to read and it is also easier to add
support for new encodings.
- A bunch of new encodings and encoding aliases have been added.
- Support for 1->2, 1->3 and 1->4 mappings, which is needed for
transliterating with flying accents as GNU does, like "u.
- Lots of warnings have been fixed, the major part of the code is
now WARNS=6 clean.
- New section 1 and section 5 manual pages have been added.
- Some GNU-specific calls have been implemented:
iconvlist(), iconvctl(), iconv_canonicalize(), iconv_open_into()
- Support for GNU's //IGNORE suffix has been added.
- The "-" argument for stdin is now recognized in iconv(1) as per POSIX.
- The Big5 conversion module has been fixed.
- The iconv.h header files is supposed to be compatible with the
GNU version, i.e. sources should build with base iconv.h and
GNU libiconv. It also includes a macro magic to deal with the
char ** and const char ** incompatibility.
- GNU compatibility: "" or "char" means the current local
encoding in use
- Various cleanups and style(9) fixes.

Approved by: delphij (mentor)
Obtained from: The NetBSD Project
Sponsored by: Google Summer of Code 2009