Screen: Improve timestamping fallback for DC1394 video capture engine.

libdc1394 doesn't zero-init the timestamp field properly, so
timestamps on the USB backend arent't nicely zeroed to signal
lack of support, but contain random trash numbers - Hard to
detect reliably if backend supports timestamps or not.

To allow running on existing, non-fixed libdc1394 versions,
use a different approach:

1. We detect if camera is Firewire or USB. Iff Firewire, use
   precise timestamps. Iff USB, use fallback.

2. Detection is a hack in itself: libdc doesn't expose the
   info about its version number or the backend in use via
   public stable api afaics - unless i overlooked it somehow.
   Therefore we check if dc1394_read_cycle_timer() is supported.
   On Firewire, this is supported, on USB it is absent atm., so
   use it as Firewire vs. USB backend indicator.

-> Screen for 64-Bit Octave on Linux rebuilt and tested against
   a IIDC Firewire cam and a IIDC USB-3 Vision cam. Works.