ofp-util: New function ofputil_port_to_string().

This function is more convenient than ofputil_format_port() when a "struct
ds" is not already in use.  This commit converts one caller for which this
was already true, and the following commit will add another.

Signed-off-by: Ben Pfaff <blp@nicira.com>
ofproto-dpif: Make "fdb/show" report OpenFlow port numbers.

Users are more likely to be able to reasonably interpret OpenFlow port
numbers than datapath port numbers.

This issue has existed since at least 2011 but only recently has it been
possible for OpenFlow and datapath port numbers to differ (except for the
"local" port).

Reported-by: Christopher Paggen <cpaggen@cisco.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>