all: pass log IDs as the proper type rather than strings

This change focuses on the backend log ID, which is the mostly commonly
used in the client.  Tests which don't seem to make use of the log ID
just use the zero value.

Signed-off-by: Will Norris <will@tailscale.com>

ipn/ipnserver/server.go

@@ -25,6 +25,7 @@ import (
 	"tailscale.com/ipn/ipnlocal"
 	"tailscale.com/ipn/localapi"
 	"tailscale.com/types/logger"
+	"tailscale.com/types/logid"
 	"tailscale.com/util/mak"
 	"tailscale.com/util/set"
 	"tailscale.com/util/systemd"
@@ -35,7 +36,7 @@ import (
 type Server struct {
 	lb           atomic.Pointer[ipnlocal.LocalBackend]
 	logf         logger.Logf
-	backendLogID string
+	backendLogID logid.PublicID
 	// resetOnZero is whether to call bs.Reset on transition from
 	// 1->0 active HTTP requests. That is, this is whether the backend is
 	// being run in "client mode" that requires an active GUI
@@ -412,9 +413,9 @@ func (s *Server) addActiveHTTPRequest(req *http.Request, ci *ipnauth.ConnIdentit
 //
 // At some point, either before or after Run, the Server's SetLocalBackend
 // method must also be called before Server can do anything useful.
-func New(logf logger.Logf, logid string) *Server {
+func New(logf logger.Logf, logID logid.PublicID) *Server {
 	return &Server{
-		backendLogID: logid,
+		backendLogID: logID,
 		logf:         logf,
 		resetOnZero:  envknob.GOOS() == "windows",
 	}