directory: don't pass fd to traverseAllIn()

This patch continues the work of the previous patch: don't pass a file
descriptor at all to traverseAllIn().  Since this fd was only used to
report "directory not found" errors, we can easily move that check to
the caller.  This is a great relief, since it removes the dependency
on a client connection from a lot of enumeration functions.

src/directory.h

@@ -64,7 +64,7 @@ Song *getSongFromDB(const char *file);
 
 time_t getDbModTime(void);
 
-int traverseAllIn(int fd, const char *name,
+int traverseAllIn(const char *name,
 		  int (*forEachSong) (Song *, void *),
 		  int (*forEachDir) (Directory *, void *), void *data);