strongswan: add strongswan-mod-socket

This a virtual package that is satisfied by either
strongswan-mod-socket-default or strongswan-mod-socket-dynamic, and is
required by the charon daemon. When neither of these packages is
installed, charon will not function.

Closes #16261, #16263 and #16367.

Signed-off-by: Noel Kuntze <noel.kuntze@thermi.consulting>
Signed-off-by: Philip Prindeville <philipp@redfish-solutions.com>
Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>