On-demand/socket activation

[YaLTeR]

Compositors like Mutter do a thing where they create and listen to a DISPLAY socket on their own, then only when a client connects, they spawn Xwayland on-demand and pass it the client fd. I'd like to do the same in niri, but with xwayland-satellite.

The advantages are:

1. niri locks/creates the socket, so it can export the right DISPLAY variable automatically.
2. If xwayland-satellite dies, the socket remains open on niri side, it can just spawn a new instance right away if a new client connects.
3. This also allows xwayland-satellite to exit when all X11 clients disconnect to conserve resources.

I'm not sure how exactly this works. Here's the relevant Mutter code: https://gitlab.gnome.org/GNOME/mutter/-/blob/fc6d79eda9395fe1151049b1b0a23974911bc1d7/src/wayland/meta-xwayland.c#L850-886 Seems like it spawns Xwayland with a few fds open, then tells it about them in commandline args. I guess this needs to be exposed in Smithay's Xwayland wrapper.