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".
diff --git a/src/ex_cmds.c b/src/ex_cmds.c
index a69a052..9c84e24 100644
--- a/src/ex_cmds.c
+++ b/src/ex_cmds.c
@@ -1743,7 +1743,7 @@
 no_viminfo(void)
 {
     /* "vim -i NONE" does not read or write a viminfo file */
-    return (use_viminfo != NULL && STRCMP(use_viminfo, "NONE") == 0);
+    return STRCMP(p_viminfofile, "NONE") == 0;
 }
 
 /*
@@ -2093,8 +2093,8 @@
 {
     if (file == NULL || *file == NUL)
     {
-	if (use_viminfo != NULL)
-	    file = use_viminfo;
+	if (*p_viminfofile != NUL)
+	    file = p_viminfofile;
 	else if ((file = find_viminfo_parameter('n')) == NULL || *file == NUL)
 	{
 #ifdef VIMINFO_FILE2