Add unicode_{column|header|border}_style to psql

 Add unicode_{column|header|border}_style to psql
 
 With the unicode linestyle, this adds support to control if the
 column, header, or border style should be single or double line
 unicode characters.  The default remains 'single'.
 
 In passing, clean up the border documentation and address some
 minor formatting/spelling issues.
 
 Pavel Stehule, with some additional changes by me.