Replaces map_to_relative_path() from Mapper.cxx.

Detach the Song class completely from the public API, only to be used
by SimpleDatabase and the update thread.

Reduces overhead because we need to manage only one memory allocation.
According to valgrind/massif, we save 7%.

From now on, struct Song will be used by the database only, and
DetachedSong will be used by everybody else.  DetachedSong is easier
to use, but Song has lower overhead.

Implements error checking for the gmtime_r() call, which was missing
in two code locations.

Use it in handle_lsinfo(), and eliminate some duplicate code.

If a song with an absolute path points inside the music directory,
print only the relative part.  This happens when partial songs from a
playlist file were loaded.

On WIN32, "%F" and "%T" are not implemented.

The only "return" statement always returns 0.

Added attributes start_ms, end_ms.  This allows us to address a
portion of a song file (important for CUE support).  There is no
support yet for storing these attributes in the state file.

After we've been hit by Large File Support problems several times in
the past week (which only occur on 32 bit platforms, which I don't
have), this is yet another attempt to fix the issue.

When song->mtime was not initialized properly, it was revealed that
strftime() might crash when gmtime_r() returns NULL due to an invalid
time_t input value.

Added the response line "Last-Modified", which sends the modification
time in ISO 8601.  The same was already implemented for playlists.

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.

Added the uri_remove_auth() library function which strips username
and password from a HTTP URI, and use it in song_print_url().  This
allows you to add HTTP URIs to the playlist including secret username
and password, without disclosing it to all MPD clients.

directory_is_root() is cheaper than
isRootDirectory(directory_get_path()).

Commit 0bfe7802 broke update for new files in the root directory,
because music_root->path was an empty string and not NULL.  There were
some NULL tests missing.  Change them to !isRootDirectory(path)
instead of path!=NULL.

CamelCase is ugly, rename the functions.

CamelCase is ugly...  rename all functions.