RaspiVid: Fix file splitting code.

File splitting code was failing to write the first set of
header bytes to all except the first file due to a bug
introduced by the circular buffer handling.
See http://www.raspberrypi.org/forums/viewtopic.php?f=43&t=76172
Merge pull request #168 from 6by9/master

RaspiVid: Fix file splitting code.
Add exposure=off option