launch: Send DEACTIVATED signal *before* disabling devices
launch: Die if setuid or setgid fails
Update README
Make view_attach return a negative error code instead of just false

This way, the compositor can appropriately handle EACCES from a failed
page flip.
compositor: Handle EACCES when view_attach fails

This also changes perform_update to add screens to the pending_flips
mask only after the attach succeeded. This way, when an attach fails, we
still try to update that screen later on.