utils: use common code for apply_rules and apply_cipso

Code for apply_rules() has been rewritten to use opendir() and readdir(),
but apply_cipso() remained implemented with nftw().
This patch implements both applying functions with opendir() and readdir()
using a common internal function apply_dir().
The resulting code is 45 lines shorter and keeps directory traversal logic
in single place.

Signed-off-by: Rafal Krypa <r.krypa@samsung.com>