libsmack: clean up smack_accesses_add_modify()

commit 22c98dd03e352057b23f8d8d2d9ad252175c685b mainline

This patch makes smack_accesses_add_modify() easier to follow by
changing variable names bit more self-documentative and less easier
to mix up with adding and deleting rules. This patch also fixes some
very minor coding style errors.

Also documentation of this function is cleaned up.