ipn/ipnlocal: use the fake peerapi listener as fallback if netstack available

The previous commit (1b89662eff) this for Android, but we can also use
this on any platform if we we would otherwise fail.

Change-Id: I4cd78b40e9e77fca5cc8e717dd48ac173101bed4
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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