Changes

osmo_io: Track IOFD_FLAG_FD_REGISTERED in all backends

This will be used in common segmentation handling code to figure out
whether the iofd is still registered or was unregistered by the user
during the read cb, in order to know whether to continue submitting read
events upwards or to discard the handling.

Change-Id: Id5e92aa22ce1c5d76028c539784118be227b9d5a