Make current_ui_out be per UI
    
    Similarly to gdb_stdout&co.
    
    gdb/ChangeLog:
    2016-06-21  Pedro Alves  <palves@redhat.com>
    
    	* top.c: Call gen_ret_current_ui_field_ptr for current_uiout.
    	* top.h (struct ui) <m_current_uiout>: New field.
    	* ui-out.c (current_uiout): Delete.
    	* ui-out.h (current_uiout): Delete.
    	(current_ui_current_uiout_ptr): New declaration.
    	(current_uiout): Reimplement as wrapper around
    	current_ui_current_uiout_ptr.