Implement Precision interface in all filesystems

* drive: add mime type on upload
  * drive: correct info on upload to fix crash
  * local: measure time precision by making a local file and using Chtimes
  * swift: move snet parameter here
  * core: add -modify-window and compute the optimum one
  * core: use modify window when checking times

fs_s3.go

@@ -244,6 +244,11 @@ func (f *FsS3) Rmdir() error {
 	return f.b.DelBucket()
 }
 
+// Return the precision
+func (fs *FsS3) Precision() time.Duration {
+	return time.Nanosecond
+}
+
 // ------------------------------------------------------------
 
 // Return the remote path