network: do not set nexthop ID, gateway, and multipath routes simultaneously

network: do not set nexthop ID, gateway, and multipath routes simultaneously

See kernel's rtm_to_fib_config() in net/ipv4/fib_frontend.c and
rtm_to_fib6_config() in net/ipv6/route.c.

Note that if both gateway and multipath routes are specified, then
kernel ignores gateway. So, strictly speaking, setting both gateway and
multipath routes is allowed by kernel. But such situation is mostly
user's misconfiguration. Let's refuse it.

Note that the conditions newly added in route_configure() are redundant,
as all static configurations are already verified in
route_section_verify(), and dynamic configurations do not set
nexthop_id or multipath routes. Just for safety.