Remove empty hourly video files before concat to avoid crashes
Avoid error of /bin/rm args line too long at remove_old.sh
Add hourly_hook-sample.sh

This script (after tuning MASTER_SERVER* constants in it) may be used in
distributed system (i.e. hourly videos are made locally, and transmitted to
master server where it concatenates it into daily pieces).
Update ffmpeg configure options for new tasks