Use empty media descriptor when rejecting SDP media lines

[bennylp]

Currently when we reject a media line in SDP offer, we copy the media descriptor from the offer in the answer and set the port to zero.

This works fine. However this may create some confusions when troubleshooting a problem, therefore using empty media descriptor (m= line with zero format) may be preferable and perhaps even more efficient.

Since this may cause interop problems as some endpoints may not allow m= line with zero formats (even though this is perfectly legal), this feature should be made configurable by compile time setting.