When there are duplicate slashes in the song paths, eliminate them;
example:

 /var/lib/mpd/music//foo.mp3

becomes:

 /var/lib/mpd/music/foo.mp3

The slash is only detected at the border between the music_directory
and the local part.

When the user configures a music_directory with a trailing slash, it
may break playlist loading, because MPD expects a double slash.  Chop
off the trailing slash.

Some code will be a little bit simpler if the dot is part of the
string literal.

Use config_get_path() instead in mapper.c.

Without a music_directory, MPD is an excellent streaming client.

Renamed functions, types, variables.

g_get_user_special_dir() was introduced with GLib 2.14.  Don't use it
in older versions.

Don't use fixed static buffers.  GLib allocates a new string for us
anyway, let's just return this one instead of copying it.

Don't use fixed stack buffers.

Don't use fixed stack buffers.

Don't pass a static buffer to map_spl_utf8_to_fs().

If no music_directory is configured, fall back to the XDG music
directory, determined by
g_get_user_special_dir(G_USER_DIRECTORY_MUSIC).

Only include headers which are really needed.  os_compat.h aimed to
make MPD easily portable, but was never actually made portable.

When save_absolute_paths_in_playlists was enabled in mpd.conf, MPD
broke all playlists when manipulated using the "playlistdelete"
command.  The reason was that map_directory_child_fs() was used, which
doesn't accept slashes in the file name.  Use the new map_uri_fs()
function instead.

In map_directory_child_fs(), the result of utf8_to_fs_charset() was
never used, because it passed the unmodified "name" parameter to
pfx_dir().

Make map_directory_child_fs() refuse the names "." and "..".  This is
currently the interface where an attacker may inject a manipulated
path (through the "update" command).

Added the function map_spl_utf8_to_fs() which replaces
utf8_to_fs_playlist_path().

g_error() is fatal by default.  Use g_warning() for non-fatal
initialization errors.

Due to an off-by-one bug in map_fs_to_utf8(), the function returned
database paths with a leading slash.

Clients which have authenticated via unix socket may add local files
to the MPD playlist, provided that they own the file.

Moved the musicDir variable and its initialization code from path.c to
mapper.c.

The mapper library maps directory and song objects to file system
paths.  With this central library, the code mixture in path.c should
be cleaned up, and we will be able to add neat features like aliasing.