ipn/ipnserver: move the unserved connection logic to a Listener

So future refactors can only deal with a net.Listener and
be unconcerned with their caller's (Windows-specific) struggles.

Change-Id: I0af588b9a769ab65c59b0bd21f8a0c99abfa1784
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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