lua loader: Add disable-device to disable a device.

disable-device fooX will set hint.foo.X.disabled=1 as a way to easily
disable a device attaching during boot.

Reviewed by: tsoome
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D31297

(cherry picked from commit 07c4b78d0a1d26b6441cf3e52b917f20c932b9d0)