I determined which options got changed by executing the following
commands in the strongswan repository:
git diff -U20 5.6.0..5.6.1 src/swanctl/swanctl.opt
git diff -U20 5.6.0..5.6.1 conf
The strongswan-swanctl systemd service starts charon-systemd. This implements a IKE daemon
very similar to charon, but it's specifically designed for use with systemd. It uses the
systemd libraries for a native integration.
Instead of using starter and an ipsec.conf based configuration, the daemon is directly
managed by systemd and configured with the swanctl configuration backend.
See: https://wiki.strongswan.org/projects/strongswan/wiki/Charon-systemd
Note that the strongswan.conf and swantctl.conf configuration files are automatically
generated based on NixOS options under services.strongswan-swanctl.strongswan and
services.strongswan-swanctl.swanctl respectively.
