Make ScriptBroker actually work on remote machines

To execute script files (i.e., .py or .lua files) on remote machines on all platforms through yarprun, I implemented a 'which' command in yarprun that checks if a given file can be found in the $PATH on the host machine, and returns its location. This allows yarpmanager to execute a script file on a remote machine; in the application xml description, the 'module' element for this script will have the filename within the 'name' tags, while the script interpreter (e.g., python, lua) should be provided within the 'deployer' tags.

@ale-git can you check that I didn't mess yarprun up?
@apaikan can you check that this works as expected?