Refactoring Python interfaces
Refactor:
- plash.pola_run_args: pass in root dir, FDs, env vars etc. as arguments, rather than taking them from environment
- plash.process: split up functionality so that we don't need to use inheritance to customise
- spawn_unix_process(cmd, args, env, fds, cwd)
- spawn_plash_process(cmd, args, env, fds, caps): two implementations, for within sandbox and for outside sandbox. Unit tests for checking that they set up arguments correctly.