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

.github/scripts/spacecheck.pl

@@ -30,7 +30,7 @@ my @tabs = (
     "^m4/zz40-xc-ovr.m4",
     "Makefile\\.[a-z]+\$",
     "/mkfile",
-    "\\.(bat|cmd|sln|vc)\$",
+    "\\.(bat|sln|vc)\$",
     "^tests/certs/.+\\.der\$",
     "^tests/data/test",
 );
@@ -43,7 +43,7 @@ my @mixed_eol = (
 
 my @need_crlf = (
     "\\.(bat|sln)\$",
-    "^winbuild/.+\\.(cmd|md)\$",
+    "^winbuild/.+\\.md\$",
 );
 
 my @space_at_eol = (