Spawns a process on a screen using argument and environment lists.
- screen
- A screen to spawn on.
- working_directory
- The process working directory.
- argv
- Argument list, with program at index 0.
- envp
- List of environment variables.
- flags
- Spawning flags.
- child_setup
- Child setup callback.
- child_pid
- Returns the child process id.
If true, the process was spawned successfully.