enable -Wpointer-arith, -Wstrict-prototypes

Also enable -Wunused-parameter - this forces us to add the gcc
"unused" attribute to a lot of parameters (mostly library callback
functions), but it's worth it during code refactorizations.

src/playlist.c

@@ -1258,7 +1258,7 @@ int previousSongInPlaylist(int fd)
 	}
 }
 
-int shufflePlaylist(int fd)
+int shufflePlaylist(mpd_unused int fd)
 {
 	int i;
 	int ri;