ofp-print: Move much of the printing code into message-specific files.

ofp-print: Move much of the printing code into message-specific files.

Until now, the ofp-print code has had a lot of logic specific to
individual messages.  This code is better put with the other code specific
to those messages, so this commit starts to migrate it.

There is more work of a similar type to do, but this is a reasonable start.

Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Justin Pettit <jpettit@ovn.org>