Keep display on and block suspend when os update is in progress

A flag file is created when os update is in progress. While the
flag file is present the display should stay on and the device should
not suspend.

[mce] Keep display on and block suspend when os update is in progress. Fixes JB#14675
Block tklock while update mode is active

If update mode gets activated while home screen is available,
tklock should be blocked.
Re-send current tklock state if change request is denied

If tklock change request can't be fulfilled, the active state needs to
be signaled over D-Bus so that the other parties are not left assuming
that the request went through.
Move extern datapipe declarations from mce.h to datapipe.h

The actual variables and related init/quit logic is already in datapipe.c.
Do not send shutdown/reboot requests to dsme when update mode is active
Merge pull request #179 from spiiroin/update_mode

Update mode