The source project of this merge request has been removed.
While working on [1] and [2] I noticed that more than CAP_SETPCAP was needed to have the strongswan and strongswan-swanctl services working with priv downgrade. With this merge request, both services worked in my tests.
1: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=927961 2: https://bugs.launchpad.net/debian/+source/strongswan/+bug/1826238