ipsec-interface using sec(4) on OpenBSD

This test first demonstrates a configuration using IKED, followed by
the equivalent behaviour using libreswan (at least in theory).

With IKED:

- the device sec1 is created with:
  - the local child's CIDR
  - the remote child's gateway
  - made up

- IKED then establishes an IPsec tunnel adding kernel state (SADBs)
  bound to the interface (there is no kernel policy!)

- Note: for what ever reason the sec(4) device doesn't send ping
  replies.  IPsec stats and a tcpdump of sec(4) show the packets flow
  from one end to the other BUT the sec(4) device doesn't send back an
  ICMP response.

See https://man.openbsd.org/sec

The sec(4) device was added to OpenBSD in 2024.
