ssh-agent plumbing on the nassh side.

The NaCl plugin bubbles up connect requests for ssh-agent which this cl relays
towards the application id as present in SSH_AUTH_SOCK environment variable.

BUG=chromium:211948
TEST=manual
Change-Id: I6ebe361ad696aa1a59ee89ee018aca81c6b0fe5b
Reviewed-on: https://chromium-review.googlesource.com/195575
Reviewed-by: Robert Ginda <rginda@chromium.org>
Tested-by: Robert Ginda <rginda@chromium.org>