Added ffmpeg fallback when avconv is not available

On some distros, avconv isn't available yet.
This fix adds a fallback to ffmpeg.
When none is found, no error message is displayed in the console any
more.

Fixes #5745

Also, this now uses exec() instead of shell_exec() to be able to get the
return code.

The binary path found with "which" is now used to prevent further FS
binary lookup when running the commands.

Moved the "-ss" parameter before "-i" for ffmpeg.
Fixed the rebellous space, replaced with a tab
Fixed undefined variable error when using avconv
Added missing escaping
Merge branch 'master' into moviepreviewfallback

Conflicts:
	lib/private/preview/movies.php
Merge pull request #5767 from owncloud/moviepreviewfallback

Added ffmpeg fallback when avconv is not available