Review: Fix regression that did not allow "route change" that didn't re-specify the gateway

[Ryan Stone]

A change a while back (r264986) broke the ability to change a route's
parameters if the gateway wasn't explicitly specified again.  For
example, you used to be able to run "route change 10.0.0.0/8 -mtu
9000" to change the MTU without having to provide the gateway IP to
the command.  I have a review open to restore the existing behaviour.
In cases where the gateway is not specified, the gateway on the route
remains unchanged.

Anyone interested in reviewing this change should add themselves as a
reviewer to https://reviews.freebsd.org/D14291.

Thanks,
Ryan