Enum arguments (ARG_ENUM with .list != LST_bool) are assumed to be of
type/size int in assign_args() in args.c.
Fixes: 0644ebd3de62 ("implemented IKE_SA uniqueness using ipsec.conf uniqueids paramater additionally supports a "keep" value to keep the old IKE_SA")
Closes strongswan/strongswan#2148