patch 8.2.3908: cannot use a script-local function for 'foldtext'

Problem:    Cannot use a script-local function for 'foldtext'.
Solution:   Expand "s:" and "<SID>". (Yegappan Lakshmanan, closes #9411)
diff --git a/src/optionstr.c b/src/optionstr.c
index c69354e..ae584d0 100644
--- a/src/optionstr.c
+++ b/src/optionstr.c
@@ -2310,6 +2310,7 @@
 # endif
 # ifdef FEAT_FOLDING
 	    varp == &curwin->w_p_fde ||
+	    varp == &curwin->w_p_fdt ||
 # endif
 	    gvarp == &p_fex ||
 # ifdef FEAT_FIND_ID
@@ -2341,8 +2342,10 @@
 	    p_opt = &p_dex;
 # endif
 # ifdef FEAT_FOLDING
-	if(varp == &curwin->w_p_fde)	// 'foldexpr'
+	if (varp == &curwin->w_p_fde)	// 'foldexpr'
 	    p_opt = &curwin->w_p_fde;
+	if (varp == &curwin->w_p_fdt)	// 'foldtext'
+	    p_opt = &curwin->w_p_fdt;
 # endif
 	if (gvarp == &p_fex)	// 'formatexpr'
 	    p_opt = &curbuf->b_p_fex;