wam/wash/saltpig: Add executeContext.requestTTY

* Add a executeContext.requestTTY method so that the thing being executed
  can request changes to the TTY object.
* Make saltpig use this to request that keyboard interupts be disabled
  once the nacl plugin is running.

Change-Id: I845d6835d9de5af0e28984914ada68bc06f4431d
Reviewed-on: https://chromium-review.googlesource.com/199422
Reviewed-by: Robert Ginda <rginda@chromium.org>
Tested-by: Robert Ginda <rginda@chromium.org>