Send the spooled PostScript to the printer.
Interactively, when you use a prefix argument (C-u), the command prompts the
user for a file name, and saves the spooled PostScript image in that file
instead of sending it to the printer.
Noninteractively, the argument FILENAME is treated as follows: if it is nil,
send the image to the printer. If FILENAME is a string, save the PostScript
image in a file with that name.
(fn &optional FILENAME)