Missing from commit 48c96bba

The root WatchDirectory instance already contains the music directory,
and we don't need to fetch it again.

Simplifies a lot of code, because we don't need to return both the
SongEnumerator and the InputStream.

Replaces map_to_relative_path() from Mapper.cxx.

Move code from Path::RelativeFS() and make it generic.

Don't use the mapper - all DetachedSong instances we get have already
been mapped.

The DetachedSong instance already knows its own absolute path, and
there is no need to ask the mapper again.

Hide inside CreateLocalStorage().

Use Storage::MapUTF8() internally, don't use global variables.

libsmbclient is not thread-safe nor reentrant.  We must protect all
function calls with a global mutex, unfortunately.

Since the encoder plugin uses a libvorbis function (and not only
libvorbisenc functions), we need to link with libvorbis explicitly.

Passing a void pointer is unsafe.

This commit finally allows the database update to scan remote files,
e.g. using the smbclient storage plugin.  However, it is not yet
possible to configure that, therefore the feature is not accessible
yet.