We were inconsistent whether we checked if the feature was already
enabled which we could do cheaply using the locally available status.
We would do the checks fine if we were turning on funnel, but not serve.
This moves the cap checks down into enableFeatureInteractive so that
are always run.
Updates #9984
Co-authored-by: Tyler Smalley <tyler@tailscale.com>
Signed-off-by: Maisem Ali <maisem@tailscale.com>