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
Fixed default struct initialization because I write C++ normally
Fixed bug where uniproc_win64.c would not correctly append arguments to the command-line string
Modified uniproc_create_process signature to take a command and a list of arguments
Updated uniproc_win64.c to terminate the process before closing the handles
Updated uniproc_win64.c to convert the command and list of arguments into a command-line string
