Screen/dc1394: Minor improvements to dc1394 capture engine.

More debug output for (non-)Format-7 setup code.
Accept capturerate DBL_MAX aka realmax in Format-7 mode.

-> Lot's of mostly unsucessfull attempts at improving fps in
   Format-7 mode.

-> Don't get any framerates even remotely close to what the
   vendor claims, at least not on this tested controller.

-> Framerate reporting in Format-7 is completely bogus.

-> At least we now know that non-Format-7 works correctly
   but provides lower fps because the camera tells something
   else than what the manual claims.

-> At least Format-7 modes now mostly work, although at too
   low fps. And full resolution on the Flea is achieved.

   Key is to set usbcore.usbfs_memory_mb=1000 at Linux bootloader
   or during runtime as module parameter. The default of 16 MB
   causes out of memory URB allocation failures for higher resolution
   video modes.

-> Screen for 64-Bit Octave on Linux rebuilt and tested.