fsckd daemon for inter-fsckd communication

fsckd daemon for inter-fsckd communication

Global logic:
Add systemd-fsckd multiplexer which accepts multiple (via systemd-fsck's
/run/systemd/fsck.progress socket) 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.

Plymouth and user interaction:
Forward the progress to plymouth and support canellation of in progress fsck.
Try to connect and send to plymouth (if running) some checked report progress,
using direct plymouth protocole.

Update message is the following:
fsckd:<num_devices>:<progress>:<string>
* num_devices corresponds to the current number of devices being checked (int)
* progress corresponds to the current minimum percentage of all devices being
  checked (float, from 0 to 100)
* string is a translated message ready to be displayed by the plymouth theme
  displaying the information above. It can be overridden by plymouth themes
  supporting i18n.

Grab in fsckd plymouth watch key Control+C, and propagate this cancel request
to systemd-fsck which will terminate fsck.

Send a message to signal to user what key we are grabbing for fsck cancel.

Message is: fsckd-cancel-msg:<string>
Where string is a translated string ready to be displayed by the plymouth theme
indicating that Control+C can be used to cancel current checks. It can be
overridden (matching only fsckd-cancel-msg prefix) for themes supporting i18n.

Misc:
systemd-fsckd stops on idle when no fsck is connected.
Add man page explaining the plymouth theme protocol, usage of the daemon
as well as the socket activation part. Adapt existing fsck man page.

Note that fsckd had lived in the upstream tree for a while, but was removed.
More information at
http://lists.freedesktop.org/archives/systemd-devel/2015-April/030175.html
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Gbp-Pq: Topic debian
Gbp-Pq: Name fsckd-daemon-for-inter-fsckd-communication.patch