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/option.h

@@ -920,6 +920,7 @@ EXTERN char_u	*p_fcs;		/* 'fillchar' */
 #endif
 #ifdef FEAT_VIMINFO
 EXTERN char_u	*p_viminfo;	/* 'viminfo' */
+EXTERN char_u	*p_viminfofile;	/* 'viminfofile' */
 #endif
 #ifdef FEAT_SESSION
 EXTERN char_u	*p_vdir;	/* 'viewdir' */