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>
diff --git a/tools/bpf/bpftool/Documentation/bpftool.rst b/tools/bpf/bpftool/Documentation/bpftool.rst
index f547a0c..6732a5a 100644
--- a/tools/bpf/bpftool/Documentation/bpftool.rst
+++ b/tools/bpf/bpftool/Documentation/bpftool.rst
@@ -16,7 +16,7 @@
 
 	**bpftool** **version**
 
-	*OBJECT* := { **map** | **program** }
+	*OBJECT* := { **map** | **program** | **cgroup** }
 
 	*OPTIONS* := { { **-V** | **--version** } | { **-h** | **--help** }
 	| { **-j** | **--json** } [{ **-p** | **--pretty** }] }
@@ -28,6 +28,8 @@
 	*PROG-COMMANDS* := { **show** | **dump jited** | **dump xlated** | **pin**
 	| **load** | **help** }
 
+	*CGROUP-COMMANDS* := { **list** | **attach** | **detach** | **help** }
+
 DESCRIPTION
 ===========
 	*bpftool* allows for inspection and simple modification of BPF objects
@@ -53,4 +55,4 @@
 
 SEE ALSO
 ========
-	**bpftool-map**\ (8), **bpftool-prog**\ (8)
+	**bpftool-map**\ (8), **bpftool-prog**\ (8), **bpftool-cgroup**\ (8)