ofp-util: Separate output, error reporting in ofputil_port_from_string().

ofp-util: Separate output, error reporting in ofputil_port_from_string().

When I wrote this function I didn't think that port 0 was important (it's
not a valid OpenFlow port number) so I used a return value of 0 to indicate
an error.  However, my assumption turns out to be wrong, so this commit
changes the interface to use the return value only for error reporting
and store the parsed port number into a pointer passed in as a parameter.

This commit doesn't change the behavior of ofputil_port_from_string().

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