Fix configured arguments for spawned shells being ignored when `use-fork = true`

[karcsesz]

Currently when use-fork is set to true in the configuration, the created processes don't receive the correct command line arguments.

This is most noticeable on linux with the default configuration, as triggering OpenConfigEditor doesn't open the configuration, just opens vi with a scratch buffer.

The fix should be rather simple, just plumbing the parameters through teletypewriter::unix::create_pty_with_fork and default_shell_command