song: replaced all song constructors

Provide separate constructors for creating a remote song, a local
song, and one for loading data from a song file.  This way, we can add
more assertions.

src/update.c

@@ -201,7 +201,7 @@ addToDirectory(struct directory *directory, const char *name)
 		struct song *song;
 		const char *shortname = mpd_basename(name);
 
-		if (!(song = newSong(shortname, directory)))
+		if (!(song = song_file_load(shortname, directory)))
 			return -1;
 		songvec_add(&directory->songs, song);
 		LOG("added %s\n", name);