ipn/ipnlocal, util/syspolicy: make run exit node a preference option

Previously, the "RunExitNode" policy merely controlled the visibility of
the "run as exit node" menu item, not the setting itself. This migrates
that setting to a preference option named "AdvertiseExitNode".

Updates ENG-2138

Change-Id: Ia6a125beb6b4563d380c6162637ce4088f1117a0
Signed-off-by: Adrian Dewhurst <adrian@tailscale.com>
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