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>