Fixes#6400
open up GETs for localapi serve-config to allow read-only access to
ServeConfig
`tailscale status` will include "Funnel on" status when Funnel is
configured. Prints nothing if Funnel is not running.
Example:
$ tailscale status
<nodes redacted>
# Funnel on:
# - https://node-name.corp.ts.net
# - https://node-name.corp.ts.net:8443
# - tcp://node-name.corp.ts.net:10000
Signed-off-by: Shayne Sweeney <shayne@tailscale.com>
