flow: Add some L7 payload data to most L4 protocols that accept it.

flow: Add some L7 payload data to most L4 protocols that accept it.

This makes traffic generated by flow_compose() look slightly more
realistic.  It requires lots of updates to tests, but at least the tests
themselves should be slightly more realistic too.

At the same time, add --l7 and --l7-len options to ofproto/trace to allow
users to specify the amount or contents of payloads that they want.

Suggested-by: Brad Cowie <brad@cowie.nz>
Signed-off-by: Ben Pfaff <blp@ovn.org>
Tested-by: Yifeng Sun <pkusunyifeng@gmail.com>
Reviewed-by: Yifeng Sun <pkusunyifeng@gmail.com>