The "pure" and "const" attributes are not so well-defined, and a
recent clang version implements an optimization which pushes the
definition's boundary beyond what I believed it was.  clang now
assumes that functions declared "pure" cannot throw exceptions, even
if they lack the "noexcept" specification.

When compiled with this new clang version, MPD will crash randomly if
an exception happens to get thrown by such as "pure" function
(https://github.com/MusicPlayerDaemon/MPD/issues/41).

This commit removes all such misplaced "pure" and "const" attributes,
closing #41.

We don't need this anymore: Win32 doesn't use this library at all, and
everything else has either iconv() or libicu.

Prepare for a ICU-based backend without GLib.

Don't expose pointer that requires the caller to invoke g_free(),
because that's GLib-only.

Don't abort the process, let the caller decide instead.

If fs_charset is empty, i.e. we're using the default "utf-8",
GetFSCharset() should return exactly that instead of an empty
std::string.

Move the "extern" declarations from output_list.c, for more type
safety.

This updates the copyright header to all be the same, which is
pretty much an update of where to mail request for a copy of the GPL
and the years of the MPD project. This also puts all committers under
'The Music Player Project' umbrella. These entries should go
individually in the AUTHORS file, for consistancy.