Launch a new process. init_func is called immediately after calling
fork(), and before closing the file descriptors and executing the program in
the new process.
file_name :
Name of the executable.
argv :
NULL-terminated parameter list.
options :
init_func :
Function to be called before launching the process.
init_data :
Value to pass to init_func.
callback :
Function to invoke when the process die.
callback_data :
Data to pass to callback when the process dies.
Returns :
An opaque structure describing the launched process.
