Security fix: mod_ffmpeg target used popen to run ffmpeg, change it to use pipe,...

Security fix: mod_ffmpeg target used popen to run ffmpeg, change it to use pipe, fork and exec. SIF files with a malicious <name> parameter or with a strange filename could previously have executed arbitrary shell commands.

git-svn-id: http://svn.voria.com/code@1181 1f10aa63-cdf2-0310-b900-c93c546f37ac