winbuild: MS-DOS batch tidy-ups

- prefer `.bat` extension over `.cmd` for MS-DOS batch, which also
  avoids confusion with OS/400 `.cmd` files.
- cleanup `echo` quotes, drop them consistently.
- delete empty output line from one of the error branches.
- prefer lowercase commands like the rest of MS-DOS batches.
- delete a contraction.
- drop backticks from error message.
- use `nmake.exe` consistently.
- use equal/not-equal operator style consistently.
- inline a single-line `if` branch.
- delete exceptions and rules dealing with Windows `.cmd` extension.

Closes #14084

packages/OS400/.gitattributes

@@ -1,6 +0,0 @@
-# Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
-#
-# SPDX-License-Identifier: curl
-
-# OS400 .cmd files are not windows scripts.
-*.cmd text eol=auto