This is the backend for the upcoming "mount" command.

Implements the error checks missing in commit 0bad8406

Build the UTF-8 version of the path automatically in the constructor.

Eliminate duplicate variable.

Move call from Mapper.cxx.

Move call from Mapper.cxx.

Don't use the obsolete Mapper library to create it.

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.