Add --sout-asf-bitrate-override setting to ASF muxer

Add --sout-asf-bitrate-override setting to ASF muxer

This patch adds setting to override calculated bit rate outputted
into ASF stream. This is needed in the cases where ASF muxer
receives stream in TS (for example) and can't correctly determine
what output bit rate should be.

Windows Media Player actually uses ASF bitrate setting to calculate
how much to cache and if the output bit rate is wrong it tries to
cache too much or too little.

To test the behaviour before this patch try to stream WMV in TS
container to another VLC that remuxes it into ASF. Open the remuxed
stream into WMP and you'll see how it tries to cache 1%, 2%, 3%
and so on...

Signed-off-by: Antoine Cellerier <dionoea@videolan.org>