implement for the "load" command the same logic used for the "add"
command: local clients can load playlist specified as absolute paths.
For relative paths the old logic is preserved: first look for a stored
playlist, then look in the music directory.
This adds a bit of overhead, but also adds flexibility to the API,
because arbitrary triggers may be invoked from that virtual method
implementation, not just Cond::signal().
The motivation for this is to make the handlers more dynamic, for the
upcoming buffering class utilizing ProxyInputStream.
Skipping those songs silently will confuse the client, because
commands specifying the song index within a playlist
(e.g. playlistdelete) will be out of sync.
This copies spl_print()'s behavior to playlist_file_print().
