By labelling devices with ENV{WL_SEAT} in udev rules the devices will be
pulled into multiple weston seats.
As a result you can get multiple independent seats under the DRM and
fbdev backends.
And as a result of this stop iterating through the compositor seat list
(of one item) and instead access the udev_input structure directly.
This enables a refactoring to pull out the weston_seat into a separate
structure permitting multiple seats.
We always call enable_udev_monitor and add_devices together and always
disable_udev_monitor and remove_devices together. Let's just have one
entry point for enable and one for disable.
