ofp-print: Merge ofp_print_stats_{reply, request}()

Merge ofp_print_stats_reply() and ofp_print_stats_request()
into a single new function, ofp_print_stats().

For stats replies there should be no run-time change in behaviour.

For pre-OpenFlow1.3 stats requests there should also be
no run-time change in behaviour.

For OpenFlow1.3+ stats requests the more flag is now printed
as present. Previously ***unknown flags 0x0001*** was printed.

Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Ben Pfaff <blp@nicira.com>