Add --foreground option

[DaanDeMeyer]

This option creates a new process group for the child process and if there's a controlling terminal, makes the new process group the foreground process group of the controlling terminal. This makes sure that SIGINT signals generated by CTRL+C are only sent to the child process and not to bubblewrap itself.

While this can also be achieved with external utilities (e.g. https://github.com/util-linux/util-linux/pull/2445), it still makes sense to support this natively in bubblewrap to avoid the need to install any other utilities in the sandbox to be able to make the child process the foreground process group of the controlling terminal.

Fixes #369