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.c

@@ -2981,6 +2981,15 @@ static struct vimoption options[] =
 			    {(char_u *)"", (char_u *)"'100,<50,s10,h"}
 # endif
 #endif
+#else
+			    (char_u *)NULL, PV_NONE,
+			    {(char_u *)0L, (char_u *)0L}
+#endif
+			    SCRIPTID_INIT},
+    {"viminfofile", "vif",  P_STRING|P_ONECOMMA|P_NODUP|P_SECURE|P_VI_DEF,
+#ifdef FEAT_VIMINFO
+			    (char_u *)&p_viminfofile, PV_NONE,
+			    {(char_u *)"", (char_u *)0L}
 #else
 			    (char_u *)NULL, PV_NONE,
 			    {(char_u *)0L, (char_u *)0L}