Fix bug #18745 with invoking Windows batch files with embedded whitespace.

 src/w32proc.c (create_child): If calling a quoted batch file,
 pass NULL for exe.

 nt/cmdproxy.c (batch_file_p): New function.
 (spawn): If calling a quoted batch file pass NULL for progname.

 test/automated/process-tests.el (process-test-quoted-batfile): New test.