core: implement RestrictNetworkInterfaces=

This commit introduces all the logic to load and attach the BPF
programs to restrict network interfaces when a unit specifying it is
loaded.

Signed-off-by: Mauricio Vásquez <mauricio@kinvolk.io>

src/basic/cgroup-util.c

@@ -2214,6 +2214,7 @@ static const char *const cgroup_controller_table[_CGROUP_CONTROLLER_MAX] = {
         [CGROUP_CONTROLLER_BPF_DEVICES] = "bpf-devices",
         [CGROUP_CONTROLLER_BPF_FOREIGN] = "bpf-foreign",
         [CGROUP_CONTROLLER_BPF_SOCKET_BIND] = "bpf-socket-bind",
+        [CGROUP_CONTROLLER_BPF_RESTRICT_NETWORK_INTERFACES] = "bpf-restrict-network-interfaces",
 };
 
 DEFINE_STRING_TABLE_LOOKUP(cgroup_controller, CGroupController);