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/ex_cmds.c b/src/ex_cmds.c
index d7a4187..d1068c7 100644
--- a/src/ex_cmds.c
+++ b/src/ex_cmds.c
@@ -3399,7 +3399,7 @@
 		|| (buf->b_flags & BF_READERR))
 	    && !p_wa
 #ifdef FEAT_QUICKFIX
-	    && !bt_nofile(buf)
+	    && !bt_nofilename(buf)
 #endif
 	    && vim_fexists(ffname))
     {