The "public key moved" panic has caused confusion on multiple occasions,
and is a known issue for Mullvad. Add a loose heuristic to detect
Mullvad nodes, and trigger distinct panics for Mullvad and non-Mullvad
instances, with a link to the associated bug.
When this occurs again with Mullvad, it'll be easier for somebody to
find the existing bug.
If it occurs again with something other than Mullvad, it'll be more
obvious that it's a distinct issue.
Updates tailscale/corp#27300
Change-Id: Ie47271f45f2ff28f767578fcca5e6b21731d08a1
Signed-off-by: Alex Chan <alexc@tailscale.com>