ofproto: Limit OVS-assigned port numbers to 32767 and below.

A couple of controller vendors have mentioned to me that they would like to
have some part of the OpenFlow port number space reserved for the
controller to use.  This commit reserves 32768 and up (roughly the upper
half of the OF1.0 port range) to the controller.

Bug #18753.
Signed-off-by: Ben Pfaff <blp@nicira.com>