asfdemux: scan last packet to determine duration for files saved from broadcasts

ASF files that were saved from broadcast may not have a duration, and
most certainly not a seek table, so try to scan the last packet and
determine an approximate duration based on the send time of the first
and last packet. Seeking still needs fixing though.

gstreamer/gst-plugins-ugly#4
15 jobs for asfdemux-scan-duration in 28 minutes and 15 seconds (queued for 12 minutes and 3 seconds)
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