patch 8.0.0716: not easy to start Vim cleanly

Problem:    Not easy to start Vim cleanly without changing the viminfo file.
            Not possible to know whether the -i command line flag was used.
Solution:   Add the --clean command line argument.  Add the 'viminfofile'
            option.  Add "-u DEFAULTS".

src/globals.h

@@ -1036,7 +1036,6 @@ EXTERN int	skip_redraw INIT(= FALSE);  /* skip redraw once */
 EXTERN int	do_redraw INIT(= FALSE);    /* extra redraw once */
 
 EXTERN int	need_highlight_changed INIT(= TRUE);
-EXTERN char_u	*use_viminfo INIT(= NULL);  /* name of viminfo file to use */
 
 #define NSCRIPT 15
 EXTERN FILE	*scriptin[NSCRIPT];	    /* streams to read script from */