core: refactor bpf firewall support into a pseudo-controller

The idea is to introduce a concept of bpf-based pseudo-controllers
to make adding new bpf-based features easier.

src/basic/cgroup-util.c

@@ -2767,6 +2767,7 @@ static const char *cgroup_controller_table[_CGROUP_CONTROLLER_MAX] = {
         [CGROUP_CONTROLLER_MEMORY] = "memory",
         [CGROUP_CONTROLLER_DEVICES] = "devices",
         [CGROUP_CONTROLLER_PIDS] = "pids",
+        [CGROUP_CONTROLLER_BPF_FIREWALL] = "bpf-firewall",
 };
 
 DEFINE_STRING_TABLE_LOOKUP(cgroup_controller, CGroupController);