udev: Introduce uaccess for remote graphical sessions (#38516)

When systemd is compiled with group-render-mode=0660, only the active
seat gets access to the render devices through uaccess. Remote desktop
sessions like gnome-remote-desktop would be left with no hardware
rendering, because those sessions are not associated with a seat.

We solve the issue by granting uaccess to specifically tagged devices on
session start, if the session is marked with
XDG_SESSION_EXTRA_DEVICE_ACCESS.

udev-builtin-uaccess is refactored to grant multiple users access to a
device, taking into account the device's seat and all the active
EXTRA_DEVICE_ACCESS sessions.