lavf/http: remove Mozilla/5.0 from user agent.

It is notably known to break playback on http streaming servers who use
the user agent to guess if it's a browser (to display a summary) or a
player (to stream the audio data).

This reverts 1fabd95.

Fixes Ticket #2663.
jpeg2000dec: make cblk->length(inc)/data use consistent

Fixes Ticket2612

Reviewed-by: Nicolas BERTRAND <nicoinattendu@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>