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 con

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

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