Use built in aac instead of libvo_aac, and other misc changes

http://ffmpeg.org/trac/ffmpeg/wiki/AACEncodingGuide says that the built in
yields better results than libvo_aac.  To use the built in, the extra -strict
options were required.  Had I known that, I probably would have used it in the
first place when replacing libfaac.

(I'm assuming that libfaac isn't being compiled against by distros because of
licensing issues?)

Given that I would have discovered the -strict option earlier if warnings were
enabled, I added a mechanism to control the log output as well as the null
piping.