ipn/ipnlocal: add logging and locking to c2n /update (#9290)

Log some progress info to make updates more debuggable. Also, track
whether an active update is already started and return an error if
a concurrent update is attempted.

Some planned future PRs:
* add JSON output to `tailscale update`
* use JSON output from `tailscale update` to provide a more detailed
  status of in-progress update (stage, download progress, etc)

Updates #6907

Signed-off-by: Andrew Lytvynov <awly@tailscale.com>

clientupdate/clientupdate.go

@@ -128,8 +128,8 @@ func NewUpdater(args Arguments) (*Updater, error) {
 			return nil, err
 		}
 	}
-	if args.PkgsAddr == "" {
-		args.PkgsAddr = "https://pkgs.tailscale.com"
+	if up.Arguments.PkgsAddr == "" {
+		up.Arguments.PkgsAddr = "https://pkgs.tailscale.com"
 	}
 	return &up, nil
 }