Remove remnants of classful behavior in route(8).

Curently route(8) treats some addresses as network addresses:

RTA_DST: inet 10.0.0.0; RTA_NETMASK: inet 255.0.0.0; RTA_IFP: link ;
RTM_GET: Report Metrics: len 240, pid: 0, seq 1, errno 0, flags:<UP,GATEWAY,STATIC>
locks: inits:
sockaddrs: <DST,NETMASK,IFP>

10.0.0.0 255.0.0.0 link#0

Note added RTA_NETMASK in the request.

Host address from the same network is ok:

route -nv get 10.0.0.1
RTA_DST: inet 10.0.0.1
RTA_IFP: link
RTM_GET: Report Metrics: len 224, pid: 0, seq 1, errno 0, flags:<UP,GATEWAY,HOST,STATIC>
...

route to: 10.0.0.1

destination: 10.0.0.0

mask: 255.255.255.0

This change eliminates network auto-guessing part AND reading network from /etc/networks.

Reviewed By: rgrimes

Differential Revision: https://reviews.freebsd.org/D24401