Add a Reboot resource into core. (RFC code.)

For ease of testing, the actual reboot is commented out.
@sersut We just talked this out during standup, and the logical thing seems to be to simply not use `Chef::DSL::reboot_pending?` to decide whether we reboot; instead, we use _only_ the internal Chef implementation. In other words, absolutely no automatic reboots unless someone has explicitly called reboot[:request].