internal/wgopenbsd: first pass at Client.Devices implementation

[mdlayher]

This doesn't actually incorporate any of the WireGuard plumbing yet; just the calls to access the devices that belong to the "wg" group.

wgopenbsd# WGCTRL_OPENBSD_KERNEL=1 go run main.go 
interface: wg0 (OpenBSD kernel)
  public key: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=
  private key: (hidden)
  listening port: 0

interface: wg1 (OpenBSD kernel)
  public key: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=
  private key: (hidden)
  listening port: 0

interface: tun0 (userspace)
  public key: L+V9o0fNYkMVKNqsX7spBzD/9oSvxM/C7ZCZX1jLO3Q=
  private key: (hidden)
  listening port: 0

Updates #34.

[mdlayher]

I'll close this once I get a sanity check. I've opened a thread at https://groups.google.com/forum/#!topic/golang-nuts/FfasFTZvU_o.

[mdlayher]

I haven't heard anything so far, but ncon on IRC gave a read through and I'd like to keep moving, so I'll merge this and do so.