patch 9.0.1262: the did_set_string_option function is too long

Problem:    The did_set_string_option function is too long.
Solution:   Split off functionality to individual functions.  (Yegappan
            Lakshmanan, Lewis Russell, closes #11904)
diff --git a/src/spell.c b/src/spell.c
index 49ada3b..5f83322 100644
--- a/src/spell.c
+++ b/src/spell.c
@@ -4386,7 +4386,7 @@
 }
 
 /*
- * Handle side effects of setting 'spell'.
+ * Handle side effects of setting 'spell' or 'spellfile'
  * Return an error message or NULL for success.
  */
     char *