patch 9.0.1330: handling new value of an option has a long "else if" chain

Problem:    Handling new value of an option has a long "else if" chain.
Solution:   Use a function pointer. (Yegappan Lakshmanan, closes #12015)
diff --git a/src/evalfunc.c b/src/evalfunc.c
index 7f6cbac..cf30ed7 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -9940,7 +9940,7 @@
 
     if (!curwin->w_p_spell)
     {
-	did_set_spelllang(curwin);
+	parse_spelllang(curwin);
 	curwin->w_p_spell = TRUE;
     }
 
@@ -10030,7 +10030,7 @@
 
     if (!curwin->w_p_spell)
     {
-	did_set_spelllang(curwin);
+	parse_spelllang(curwin);
 	curwin->w_p_spell = TRUE;
     }