patch 9.0.1300: 'statusline' only supports one "%=" item

Problem:    'statusline' only supports one "%=" item.
Solution:   Add support for multiple "%=" items. (TJ DeVries, Yegappan
            Lakshmanan, closes #11970, closes #11965)
diff --git a/src/optionstr.c b/src/optionstr.c
index d218d56..7dfb7b7 100644
--- a/src/optionstr.c
+++ b/src/optionstr.c
@@ -587,7 +587,7 @@
 	if (!*s)
 	    break;
 	s++;
-	if (*s == '%' || *s == STL_TRUNCMARK || *s == STL_MIDDLEMARK)
+	if (*s == '%' || *s == STL_TRUNCMARK || *s == STL_SEPARATE)
 	{
 	    s++;
 	    continue;