ipn/{ipnserver,localapi},tsnet: use ipnauth.Self as the actor in tsnet localapi handlers

With #14843 merged, (*localapi.Handler).servePrefs() now requires a non-nil actor,
and other places may soon require it as well.

In this PR, we update localapi.NewHandler with a new required parameter for the actor.
We then update tsnet to use ipnauth.Self.

We also rearrange the code in (*ipnserver.Server).serveHTTP() to pass the actor via Handler's
constructor instead of the field.

Updates #14823

Signed-off-by: Nick Khyl <nickk@tailscale.com>

ipn/localapi/localapi.go

@@ -169,10 +169,9 @@ var (
 	metrics   = map[string]*clientmetric.Metric{}
 )
 
-// NewHandler creates a new LocalAPI HTTP handler. All parameters except netMon
-// are required (if non-nil it's used to do faster interface lookups).
-func NewHandler(b *ipnlocal.LocalBackend, logf logger.Logf, logID logid.PublicID) *Handler {
-	return &Handler{b: b, logf: logf, backendLogID: logID, clock: tstime.StdClock{}}
+// NewHandler creates a new LocalAPI HTTP handler. All parameters are required.
+func NewHandler(actor ipnauth.Actor, b *ipnlocal.LocalBackend, logf logger.Logf, logID logid.PublicID) *Handler {
+	return &Handler{Actor: actor, b: b, logf: logf, backendLogID: logID, clock: tstime.StdClock{}}
 }
 
 type Handler struct {