bpftool: implement cgroup bpf operations

This patch adds basic cgroup bpf operations to bpftool:
cgroup list, attach and detach commands.

Usage is described in the corresponding man pages,
and examples are provided.

Syntax:
$ bpftool cgroup list CGROUP
$ bpftool cgroup attach CGROUP ATTACH_TYPE PROG [ATTACH_FLAGS]
$ bpftool cgroup detach CGROUP ATTACH_TYPE PROG

Signed-off-by: Roman Gushchin <guro@fb.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Jakub Kicinski <jakub.kicinski@netronome.com>
Cc: Martin KaFai Lau <kafai@fb.com>
Cc: Quentin Monnet <quentin.monnet@netronome.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/bpf/bpftool/Documentation/bpftool-prog.rst

@@ -155,4 +155,4 @@ EXAMPLES
 
 SEE ALSO
 ========
-	**bpftool**\ (8), **bpftool-map**\ (8)
+	**bpftool**\ (8), **bpftool-map**\ (8), **bpftool-cgroup**\ (8)