tools: bpftool: add `bpftool prog help` as real command i.r.t exit code

Make error messages and return codes more consistent. Specifically, make
`bpftool prog help` a real command, instead of printing usage by default
for a non-recognized "help" command. Output is the same, but this makes
bpftool return with a success value instead of an error.

Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>