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/quickfix.c b/src/quickfix.c
index bc82ee0..245e20b 100644
--- a/src/quickfix.c
+++ b/src/quickfix.c
@@ -4534,7 +4534,7 @@
  * Returns OK or FAIL.
  */
     char *
-qf_process_qftf_option(void)
+did_set_quickfixtextfunc(optset_T *args UNUSED)
 {
     if (option_set_callback_func(p_qftf, &qftf_cb) == FAIL)
 	return e_invalid_argument;