ofp-util: Add struct ofputil_packet_out, helper functions, and use it all.

ofp-util: Add struct ofputil_packet_out, helper functions, and use it all.

This makes the ofp-util support for packet_out better match the support
that ofp-util has for other OpenFlow messages.  It also prepares for an
upcoming patch that adds a new piece of code that generates packet_out
messages.

Signed-off-by: Ben Pfaff <blp@nicira.com>