xfsprogs/io: add readdir command

xfsprogs/io: add readdir command

readdir reads the directory entries from an open directory from
the provided offset (or 0 if not specified). On completion,
readdir prints summary information regarding the number of
operations and bytes transferred. Options are available to specify
the starting offset, length and verbose mode to dump directory
entry information.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>