[packagekit] [RFE] use lower iopriority for background update checking

[drago01]

Hi,

Currently packagekit check for updates right after login which results
into high disk usage.
This leads to a noticeable slowdown. (After login the user wants to
start apps so its even worse at this time).
A solution would be to use a lower iopriority for the background
update checking process.
This can be done by either using the SYS_ioprio_set syscall or better
ioprio_set.

It does not really matter if the update notification shows up some
seconds later.