ip route: error out on multiple via without nexthop keyword

ip route: error out on multiple via without nexthop keyword

To specify multiple nexthops in a route the user is expected to use the
"nexthop" keyword which ip route uses to create the RTA_MULTIPATH.
However, ip route always accepts multiple 'via' keywords where only the
last one is used in the route leading to confusion. For example, ip
accepts this syntax:
    $ ip ro add vrf red  1.1.1.0/24 via 10.100.1.18 via 10.100.2.18

but the route entered inserted by the kernel is just the last gateway:
    1.1.1.0/24 via 10.100.2.18 dev eth2

which is not the full request from the user. Detect the presense of
multiple 'via' and give the user a hint to add nexthop:

    $ ip ro add vrf red  1.1.1.0/24 via 10.100.1.18 via 10.100.2.18
    Error: argument "via" is wrong: use nexthop syntax to specify multiple via

Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>