execute_process: Fix invocation of .cmd/.bat with spaces

The `CreateProcessW` documentation states "to run a batch file, you must
start the command interpreter".  Use `cmd /c the.bat` to run batch files.
Also, use a "short path" to the `.bat` file if needed to avoid spaces.

Previously this worked in some cases only due to undocumented behavior
of `CreateProcessW` when given a `.bat` file.

Fixes: #26655