HPCC-9226 CmdProcess lockup on failure, and does not use path

CmdProcess will lock indefinitely if the specified process cannot be loaded.
This is quite likely as the path is not searched.

Fixes HPCC-9226 and HPCC-9227

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #4324 from richardkchapman/cmdProcess

HPCC-9226 CmdProcess lockup on failure, and does not use path

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>