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>