patch 8.1.1547: functionality of bt_nofile() is confusing

Problem:    Functionality of bt_nofile() is confusing.
Solution:   Split into bt_nofile() and bt_nofilename().
diff --git a/src/quickfix.c b/src/quickfix.c
index 290f6bc..d2d3302 100644
--- a/src/quickfix.c
+++ b/src/quickfix.c
@@ -4135,7 +4135,7 @@
     // Set the options for the quickfix buffer/window (if not already done)
     // Do this even if the quickfix buffer was already present, as an autocmd
     // might have previously deleted (:bdelete) the quickfix buffer.
-    if (curbuf->b_p_bt[0] != 'q')
+    if (bt_quickfix(curbuf))
 	qf_set_cwindow_options();
 
     // Only set the height when still in the same tab page and there is no