tools: bpftool: alias show and list commands

iproute2 seems to accept show and list as aliases.
Let's do the same thing, and by allowing both bring
cgroup syntax back in line with maps and progs.

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>