ffserver: fix seeking with ?date=...

Regression since 5f847bf6.

After this commit, timestamps pushed by FFmpeg won't be relative
anymore, but absolute (based on the date/time at the beginning of the
push). This will allow seeking to work properly. Before this patch, the
seek was done, but ffm timestamps were way smallers than the absolute
requested timestamp (based on a date), so the seek was done, but to the
end of the stream (which was similar to no effect at all).