Added function to check if all processes are finished without blocking
Added a function to set a uniproc_process object to a known invalid value
Added a function to check if a process is a known invalid value
Added userdata variable to struct as well as functions to set/get it
AMEND: Updated uniproc_close_process to set the process to a known invalid value
All functions now ignore uniproc_process's with a known invalid value
uniproc_create_process will now return a known invalid value if it fails to create a process
