ipn/ipnlocal: move nodeBackend methods from local.go to node_backend.go

We previously kept these methods in local.go when we started moving node-specific state
from LocalBackend to nodeBackend, to make those changes easier to review. But it's time
to move them to node_backend.go.

Updates #cleanup
Updates tailscale#12614

Signed-off-by: Nick Khyl <nickk@tailscale.com>

Author: nickkhyl