setup_visible

 When spaces and tabs are displayed on the screen, they can be made visible
 to the user. They appear as dots and arrow heads respectively.

 This function toggles the white space visibility flag of the current
 buffer.

 Initially a buffers white space visibility flag is determined from its
 language definition. However this call allows it to be subsequently
 modified.

 NOTE:

 On other machines, the characters used to display white space may vary.