Add circular buffer mode.

This commit adds the functionality to set up a circular buffer in memory
which stores the last N seconds of video.  When the application is send a
signal or a key is pressed the last N seconds are saved.

Currently it seems to work well enough although there seem to be problems
with playing back some video using omxplayer when the GOP size is changed.
Merge branch 'master' of https://github.com/ghollingworth/userland

Squash commits