Get rid of global variables num_dirs, num_files, and num_bytes.

Get rid of global variables num_dirs, num_files, and num_bytes.

These were defined in adu.c but not used there at all.

In fact, create.c only used num_dirs in scan_dir() as a counter for
the directory currently being processed while the other two were not
really used at all. Make num_dirs local to scan_dir(), rename it to
current_dir_num and remove the other two.

The only "real" users of these variables are all in select.c, so move
them there and make them static.