socket: assign socket units to a default slice unconditionally

Due to the chown() logic socket units might end up with processes even
if no explicit command is defined for them, hence let's make sure these
processes are in the right cgroup, and that means within a slice.

Mount, swap and service units unconditionally are assigned to a slice
already, let's do the same here, too.

(This becomes more important as soon as the ebpf/firewall stuff is
merged, as there'll be another reason to fork off processes then)