ipn, ipn/ipnserver: add IPN state for server in use, handle explicitly

On Windows, we were previously treating a server used by different
users as a fatal error, which meant the second user (upon starting
Tailscale, explicitly or via Start Up programs) got an invasive error
message dialog.

Instead, give it its own IPN state and change the Notify.ErrMessage to
be details in that state. Then the Windows GUI can be less aggresive
about that happening.

Also,

* wait to close the IPN connection until the server ownership state
  changes so the GUI doesn't need to repeatedly reconnect to discover
  changes.

* fix a bug discovered during testing: on system reboot, the
  ipnserver's serverModeUser was getting cleared while the state
  transitioned from Unknown to Running. Instead, track 'inServerMode'
  explicitly and remove the old accessor method which was error prone.

* fix a rare bug where the client could start up and set the server
  mode prefs in its Start call and we wouldn't persist that to the
  StateStore storage's prefs start key. (Previously it was only via a
  prefs toggle at runtime)

ipn/message.go

@@ -92,6 +92,17 @@ func (bs *BackendServer) SendErrorMessage(msg string) {
 	bs.send(Notify{ErrMessage: &msg})
 }
 
+// SendInUseOtherUserErrorMessage sends a Notify message to the client that
+// both sets the state to 'InUseOtherUser' and sets the associated reason
+// to msg.
+func (bs *BackendServer) SendInUseOtherUserErrorMessage(msg string) {
+	inUse := InUseOtherUser
+	bs.send(Notify{
+		State:      &inUse,
+		ErrMessage: &msg,
+	})
+}
+
 // GotCommandMsg parses the incoming message b as a JSON Command and
 // calls GotCommand with it.
 func (bs *BackendServer) GotCommandMsg(b []byte) error {