Allow to read stdout and stderr as separate streams when spawning a process

[stcarrez]

The Util.Processses package allows to spawn a process and read the standard output and error but the current mode merges the standard output and error together (the spawned process still gets a separate standard output and standard error file descriptors).

In some situations, we don't want such merge and must be able to distinguish and read the standard output and standard error produced by the launched process as separate streams.