(rmail-mime-save): If file exists, don't try to be clever and add a

(rmail-mime-save): If file exists, don't try to be clever and add a
suffix to make a unique name, just let the user decide whether or not
to overwrite it.  If the input is a directory, write the default
filename to that directory.  (Bug#4388)
(rmail-mime-bulk-handler): Ensure the save button's 'directory property
is a filename-as-a-directory.