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/proto/buffer.pro b/src/proto/buffer.pro
index 1748fb5..81be7e4 100644
--- a/src/proto/buffer.pro
+++ b/src/proto/buffer.pro
@@ -62,6 +62,7 @@
 int bt_help(buf_T *buf);
 int bt_prompt(buf_T *buf);
 int bt_popup(buf_T *buf);
+int bt_nofilename(buf_T *buf);
 int bt_nofile(buf_T *buf);
 int bt_dontwrite(buf_T *buf);
 int bt_dontwrite_msg(buf_T *buf);