This adds support for enforcing exit node LAN access, DNS and subnet
routes.
Adding new preference policies was getting repetitive, so this turns
some of the boilerplate into a table.
Updates tailscale/corp#15585
Updates ENG-2240
Change-Id: Iabd3c42b0ae120b3145fac066c5caa7fc4d67824
Signed-off-by: Adrian Dewhurst <adrian@tailscale.com>