* graphds.c (add_edge): Intitialize edge's attached data.
	(foll_in_subgraph, dfs_fst_edge, dfs_next_edge): New function
	pointer parameter.  Call pointed function on each edge during
	graph traversing.  Skip traversing the edge when the function
	returns true.
	(graphds_dfs, graphds_scc): Ditto.
	(for_each_edge): New parameter.  Pass the new parameter to callback
	function.
	* graphds.h (skip_edge_callback): New function pointer type.
	(graphds_dfs, graphds_scc): New function pointer parameter.
	(graphds_edge_callback, for_each_edge): New parameter.

From-SVN: r248964