ipn: add methods on Prefs to get/set exit node being advertised

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This code was copied in a few places (Windows, Android), so unify it
and add tests.

Change-Id: Id0510c0f5974761365a2045279d1fb498feca11e
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

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500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f