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: NQuentin Monnet <quentin.monnet@netronome.com>
Reviewed-by: NJakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>