wl-copy: Redirect stdio to /dev/null in foreground mode too

[eNV25]

This is a workaround for https://sw.kovidgoyal.net/kitty/. By default kitty waits until all processes connected by stdio die, before closing.

The current code already redirects stdio to /dev/null in background mode. This change makes it work in foreground mode too.

[YaLTeR]

But if wl-copy is in the foreground, then it's intentional that it's blocked, isn't it? As far as I can tell, wl-clipboard also only redirects the standard fds in background mode: https://github.com/bugaevc/wl-clipboard/blob/abfd513ecceef5f33632e0f63daea92f9bbe08e2/src/wl-copy.c#L54