vconn: Move OpenFlow utility functions into new file ofp-util.c.

vconn: Move OpenFlow utility functions into new file ofp-util.c.

The main purpose of the vconn code is to ship OpenFlow messages across
network connections.  Over time a large number of utility functions related
to OpenFlow messages have also crept into vconn.c, but that's really
logically separate.  This commit breaks those functions out into a new
file.