Allow for configurable timeout

[Harvie]

Hello, i need zbarcam to exit if it does not recognize QR code in given time. Therefore i use something like
timeout 30 zbarcam /dev/video0

timeout command kills it with SIGTERM after 30 seconds if nothing happens, however if i do this often, after few cycles it leads to camera being left in some stuck state. Probably because it was not properly deinitialized when zbarcam got killed. I am not saying this is zbarcam bug, even tho i haven't checked the signal handlers. However i think it would be probably more robust if zbarcam could handle such timeout itself and did properly closed the camera after such timeout. eg.:

zbarcam --timeout=30 /dev/video0