Break data_hello and control_hello messages into multiple messages.

We want to allow multiple OpenFlow connections to a switch, so that
dpctl and other management tools can also connect to a switch that
is connecte to the controller.  These tools will want to fetch the
information in the data_hello message, but currently this can only
be done by sending a control_hello message, which changes the
switch configuration state.  So breaking the hello messages up into
multiple different messages, only some of which affect configuration
state, cures the problem.

Also, clean up the kernel datapath a bit, by adding helper functions
for allocating and resizing Generic Netlink messages that encapsulate
OpenFlow messages.