fsckd daemon for inter-fsckd communication

Add systemd-fsckd multiplexer which accepts multiple systemd-fsck
instances to connect to it and sends progress report. systemd-fsckd then
computes and writes to /dev/console the number of devices currently being
checked and the minimum fsck progress. This will be used for interactive
progress report and cancelling in plymouth.

systemd-fsckd stops on idle when no systemd-fsck is connected.

Make the necessary changes to systemd-fsck to connect to the systemd-fsckd
socket.