Free the "context" pointer in the method archive_plugin.close().

Fixes a memory leak: the "archive" input plugin opens the archive, but
never closes it.  This patch moves the responsibility for doing that
to archive_plugin.open_stream().  This is an slight internal API
change, but it is the simplest and least intrusive fix for the memory
leak.

g_free(NULL) is allowed.

The second parameter of zzip_dir_open() is a pointer.  Pass the NULL
pointer instead of 0 (with implicit conversion at compile time).

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.

Start to separate private from public input_stream API.

Make those two methods optional to implement, and let input_stream.c
provide fallbacks.  The buffer() method will be removed one day, and
there is now only one implementation left (input_curl.c).

The open_stream() method opens the input_stream.  This allows the
archive plugin to do its own initialization, and it also allows it to
use input_stream.data.  We can remove input_stream.archive now, which
was unnatural to have in the first place.