patch 8.1.0230: directly checking 'buftype' value

Problem:    Directly checking 'buftype' value.
Solution:   Add the bt_normal() function. (Yegappan Lakshmanan)

src/proto/buffer.pro

@@ -56,6 +56,7 @@ void ex_buffer_all(exarg_T *eap);
 void do_modelines(int flags);
 int read_viminfo_bufferlist(vir_T *virp, int writing);
 void write_viminfo_bufferlist(FILE *fp);
+int bt_normal(buf_T *buf);
 int bt_quickfix(buf_T *buf);
 int bt_terminal(buf_T *buf);
 int bt_help(buf_T *buf);