MAC/do: Rename rule_is_valid() => rule_applies()

This function checks whether a rule applies in the current context,
i.e., if the subject's users/groups match that of the rule. By
contrast, it doesn't check if the rule as specified by the user is valid
(i.e., consistent).

Reviewed by: bapt
Approved by: markj (mentor)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D47592