ofp-util: Add functions for working with OpenFlow 1.1 port numbers.

ofp-util: Add functions for working with OpenFlow 1.1 port numbers.

OpenFlow 1.1 extends port numbers to 32 bits.  Initially we plan to support
only port numbers in the 16-bit range in Open vSwitch.  However the OF1.1
reserved ports have high-valued fixed numbers that require translation to
high fixed values in the 16-bit range for OF1.0.  These new functions
provide this translation.

Nothing uses these functions yet.

Reviewed-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Ben Pfaff <blp@nicira.com>