vim-patch:8.0.0716: "--clean", 'shadafile' #9907

Nvim notes:
- Nvim does not support "-u DEFAULTS", that change is omitted.
- Also add 'shadafile' as an alias to 'viminfofile'.
- Deprecate 'viminfofile'.

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".
c4da113ef9

src/nvim/globals.h

@@ -754,7 +754,6 @@ 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 *used_shada_file INIT(= NULL);  // name of the ShaDa file to use
 
 EXTERN FILE *scriptout INIT(= NULL);  ///< Stream to write script to.