client: Add F_SEAL_SHRINK seal to shm pool backing file

The compositor must handle the case where the client attempts to shrink
shm pool backing file. Usually it's done by adding a SIGBUS handler that
remaps the backing file so it's filled with zeros.

libwayland-server has a clever trick to avoid doing messy signal
handling business by checking if the backing file has F_SEAL_SHRINK seal.

This change makes KWayland::Client::ShmPool more nicer to the compositor.