tka: move disablement logic out-of-band from AUMs

It doesn't make a ton of sense for disablement to be communicated as an AUM, because
any failure in the AUM or chain mechanism will mean disablement wont function.

Instead, tracking of the disablement secrets remains inside the state machine, but
actual disablement and communication of the disablement secret is done by the caller.

Signed-off-by: Tom DNetto <tom@tailscale.com>

tka/tka.go

@@ -549,6 +549,14 @@ func Bootstrap(storage Chonk, bootstrap AUM) (*Authority, error) {
 	return Open(storage)
 }
 
+// ValidDisablement returns true if the disablement secret was correct.
+//
+// If this method returns true, the caller should shut down the authority
+// and purge all network-lock state.
+func (a *Authority) ValidDisablement(secret []byte) bool {
+	return a.state.checkDisablement(secret)
+}
+
 // Inform is called to tell the authority about new updates. Updates
 // should be ordered oldest to newest. An error is returned if any
 // of the updates could not be processed.