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/option.h b/src/option.h
index c74a89b..7394c7f 100644
--- a/src/option.h
+++ b/src/option.h
@@ -920,6 +920,7 @@
 #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' */