The default is still users can debug their own nodes. But like
cd916b728b did, this adds support for admins to grant additional
capabilities with the new tailcfg.CapabilityDebugPeer cap.
Updates #4217
Change-Id: Ifce3d9a1f8e8845797970a4f97b393194663d35f
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>