mac_ipacl: Use IfAPI

Use `if_t` instead of `struct ifnet *`, and if_name() accessor.

Sponsored by: Juniper Networks, Inc.

sys: Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

mac_ipacl: new MAC policy module to limit jail/vnet IP configuration

The mac_ipacl policy module enables fine-grained control over IP address
configuration within VNET jails from the base system.
It allows the root user to define rules governing IP addresses for
jails and their interfaces using the sysctl interface.

Requested by: multiple
Sponsored by: Google, Inc. (GSoC 2019)
MFC after: 2 months
Reviewed by: bz, dch (both earlier versions)
Differential Revision: https://reviews.freebsd.org/D20967