[WireGuard] Suggestion: Hide private key by default with wg tool

[Jason A. Donenfeld]

Hi Bin,

I think I'll just get rid of it completely in `show` but not in the
more explicit views. So,

  - # wg show wg0  -- no private key
  - # wg show wg0 private-key -- yes private key
  - # wg showconf wg0 -- yes private key

What would you think of this?

Regards,
Jason