throughtput: enhance help output

Signed-off-by: Hagen Paul Pfeifer <hagen@jauu.net>
Merge branch 'master' of http://github.com/hgn/captcp
formatting issues
added --extended flag to statistic mod
added duration to statistic
changed tcp packet size statistics => payload size

counting the size of pure ACKs does not make sense to me
fixed format (removed tabs)
added tcp inter-arrival times to extended statistic
Merge branch 'master' of http://github.com/hgn/captcp
Merge branch 'master' of http://github.com/fixje/captcp into fixje-master