taildrop: lazily perform full deletion scan after first taildrop use (#10137)

Simply reading the taildrop directory can pop up security dialogs
on platforms like macOS. Avoid this by only performing garbage collection
of partial and deleted files after the first received taildrop file,
which would have prompted the security dialog window.

Updates tailscale/corp#14772

Signed-off-by: Joe Tsai <joetsai@digital-static.net>

taildrop/taildrop.go

@@ -67,8 +67,9 @@ func (id ClientID) partialSuffix() string {
 
 // ManagerOptions are options to configure the [Manager].
 type ManagerOptions struct {
-	Logf  logger.Logf
-	Clock tstime.DefaultClock
+	Logf  logger.Logf         // may be nil
+	Clock tstime.DefaultClock // may be nil
+	State ipn.StateStore      // may be nil
 
 	// Dir is the directory to store received files.
 	// This main either be the final location for the files