tools: bpftool: add bash completion for cgroup commands

Add bash completion for "bpftool cgroup" command family. While at it,
also fix the formatting of some keywords in the man page for cgroups.

Fixes: 5ccda64d

 ("bpftool: implement cgroup bpf operations")
Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>