bpftool: feature probing, change default action

When 'bpftool feature' is executed it shows incorrect help string.

test# bpftool feature
Usage: bpftool bpftool probe [COMPONENT] [macros [prefix PREFIX]]
       bpftool bpftool help

       COMPONENT := { kernel | dev NAME }

Instead of fixing the help text by tweaking argv[] indices, this
patch changes the default action to 'probe'. It makes the behavior
consistent with other subcommands, where first subcommand without
extra parameter results in 'show' action.

Signed-off-by: Prashant Bhole <bhole_prashant_q7@lab.ntt.co.jp>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>