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