ipn/localapi: refactor some cert code in prep for a move

I want to move the guts (after the HTTP layer) of the certificate
fetching into the ipnlocal package, out of localapi.

As prep, refactor a bit:

* add a method to do the fetch-from-cert-or-as-needed-with-refresh,
  rather than doing it in the HTTP hander

* convert two methods to funcs, taking the one extra field (LocalBackend)
  then needed from their method receiver. One of the methods needed
  nothing from its receiver.

This will make a future change easier to reason about.

Change-Id: I2a7811e5d7246139927bb86e7db8009bf09b3be3
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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