Allow route change requests to not specify the gateway.

Only require a gateway to be specified on a route add request. On
a route change request that does not specify the gateway, the
gateway will remain the same. This allows changing other route
parameters without having to re-specifying the gateway, like in
"route change 10.0.0.0/8 -mtu 9000".

Update the route(8) manpage to explicitly call out this usage
as being supported.

MFC after: 2 weeks
Sponsored by: Dell EMC Isilon
Reviewed By: eugen (rtsock.c change), rgrimes
Differential Revision: https://reviews.freebsd.org/D14291