NetBSD with ipsec-interface

On NetBSD with ipsecif(4):

- create ipsec1
- add tunnel describing ESP
- add inet describing what is tunneled

Now the magic!

By default, NetBSD assigns IDs sequentially starting at 16384.  This
behaviour can be changed by setting:

  net.ipsecif.use_fixed_reqid=1

causing the kernel to predictably assign interface ipsec<UNIT>, the IDs:

  IPv4: net.ipsecif.requid_base + 2 * UNIT
  IPv6: net.ipsecif.requid_base + 2 * UNIT + 1

(credit to Michael van Elst and #netbsd)
