wgengine/netstack: change netstack API to require LocalBackend

The macOS client was forgetting to call netstack.Impl.SetLocalBackend.
Change the API so that it can't be started without one, eliminating this
class of bug. Then update all the callers.

Updates #6764

Change-Id: I2b3a4f31fdfd9fdbbbbfe25a42db0c505373562f
Signed-off-by: Claire Wang <claire@tailscale.com>
Co-authored-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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