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/popupmnu.c b/src/popupmnu.c
index 09faa69..5fbb3bb 100644
--- a/src/popupmnu.c
+++ b/src/popupmnu.c
@@ -727,7 +727,7 @@
 		if (!resized
 			&& curbuf->b_nwindows == 1
 			&& curbuf->b_fname == NULL
-			&& curbuf->b_p_bt[0] == 'n' && curbuf->b_p_bt[2] == 'f'
+			&& bt_nofile(curbuf)
 			&& curbuf->b_p_bh[0] == 'w')
 		{
 		    /* Already a "wipeout" buffer, make it empty. */