tools: bpftool: rename cgroup list -> show in the code

So far we have used "show" as a keyword for listing
programs and maps.  Use the word "show" in the code
for cgroups too, next commit will alias show and list.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>