MAC/do: 'struct rule': IDs and types as 'u_int', rename fields

This is in preparation for introducing a common conversion function for
IDs and to simplify code a bit by removing the from-IDs union and not
having to introduce a new one for to-IDs in a later commit.

Reviewed by: bapt
Approved by: markj (mentor)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D47613

(cherry picked from commit 6aadc7b2ee055fba58984fec715b6e2a754f9d3e)