MAC: mac_policy.h: Declare common MAC sysctl and jail parameters' nodes

Do this only when the headers for these functionalities were included
prior to this one. Indeed, if they need to be included, style(9)
mandates they should have been so before this one.

Remove the common MAC sysctl declaration from
<security/mac/mac_internal.h>, as it is now redundant (all its includers
also include <security/mac/mac_policy.h>).

Remove local such declarations from all policies' files.

Reviewed by: jamie
Approved by: markj (mentor)
MFC after: 5 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D46903