derp, derphttp, magicsock: send new unknown peer frame when destination is unknown (#7552)

* wgengine/magicsock: add envknob to send CallMeMaybe to non-existent peer

For testing older client version responses to the PeerGone packet format change.

Updates #4326

Signed-off-by: Val <valerie@tailscale.com>

* derp: remove dead sclient struct member replaceLimiter

Leftover from an previous solution to the duplicate client problem.

Updates #2751

Signed-off-by: Val <valerie@tailscale.com>

* derp, derp/derphttp, wgengine/magicsock: add new PeerGone message type Not Here

Extend the PeerGone message type by adding a reason byte. Send a
PeerGone "Not Here" message when an endpoint sends a disco message to
a peer that this server has no record of.

Fixes #4326

Signed-off-by: Val <valerie@tailscale.com>

---------

Signed-off-by: Val <valerie@tailscale.com>

wgengine/magicsock/debugknobs_stubs.go

@@ -11,13 +11,13 @@ import "tailscale.com/types/opt"
 //
 // They're inlinable and the linker can deadcode that's guarded by them to make
 // smaller binaries.
-func debugDisco() bool              { return false }
-func debugOmitLocalAddresses() bool { return false }
-func logDerpVerbose() bool          { return false }
-func debugReSTUNStopOnIdle() bool   { return false }
-func debugAlwaysDERP() bool         { return false }
-func debugEnableSilentDisco() bool  { return false }
-func debugUseDerpRouteEnv() string  { return "" }
-func debugUseDerpRoute() opt.Bool   { return "" }
-
-func inTest() bool { return false }
+func debugDisco() bool                 { return false }
+func debugOmitLocalAddresses() bool    { return false }
+func logDerpVerbose() bool             { return false }
+func debugReSTUNStopOnIdle() bool      { return false }
+func debugAlwaysDERP() bool            { return false }
+func debugEnableSilentDisco() bool     { return false }
+func debugSendCallMeUnknownPeer() bool { return false }
+func debugUseDerpRouteEnv() string     { return "" }
+func debugUseDerpRoute() opt.Bool      { return "" }
+func inTest() bool                     { return false }