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/option_defs.h

@@ -630,6 +630,7 @@ EXTERN long p_ur;               ///< 'undoreload'
 EXTERN long p_uc;               ///< 'updatecount'
 EXTERN long p_ut;               ///< 'updatetime'
 EXTERN char_u *p_shada;         ///< 'shada'
+EXTERN char *p_shadafile;       ///< 'shadafile'
 EXTERN char_u *p_vdir;          ///< 'viewdir'
 EXTERN char_u *p_vop;           ///< 'viewoptions'
 EXTERN unsigned vop_flags;      ///< uses SSOP_ flags