bpftool: Support sysctl hook

Add support for recently added BPF_PROG_TYPE_CGROUP_SYSCTL program type
and BPF_CGROUP_SYSCTL attach type.

Example of bpftool output with sysctl program from selftests:

  # bpftool p load ./test_sysctl_prog.o /mnt/bpf/sysctl_prog type cgroup/sysctl
  # bpftool p l
  9: cgroup_sysctl  name sysctl_tcp_mem  tag 0dd05f81a8d0d52e  gpl
          loaded_at 2019-04-16T12:57:27-0700  uid 0
          xlated 1008B  jited 623B  memlock 4096B
  # bpftool c a /mnt/cgroup2/bla sysctl id 9
  # bpftool c t
  CgroupPath
  ID       AttachType      AttachFlags     Name
  /mnt/cgroup2/bla
      9        sysctl                          sysctl_tcp_mem
  # bpftool c d /mnt/cgroup2/bla sysctl id 9
  # bpftool c t
  CgroupPath
  ID       AttachType      AttachFlags     Name

Signed-off-by: Andrey Ignatov <rdna@fb.com>
Acked-by: Song Liu <songliubraving@fb.com>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>