shaprd: install route supports nexthop-seg6local (step1)

shaprd: install route supports nexthop-seg6local (step1)

In this patch, we can install seg6local routes from shapd cli. New sub
command "sharp install route $PREFIX nexthop-seg6local" is added to
install seg6local routes via ZEBRA_ROUTE_ADD. This is for the behaviour
test(topotest) to ensure SRv6 ZAPI is working fine.

NEW-CLI:
sharp install routes 1::1 nexthop-seg6local dum0 End 1
sharp install routes 2::1 nexthop-seg6local dum0 End_X 2001::1 1
sharp install routes 3::1 nexthop-seg6local dum0 End_T 10 1
sharp install routes 4::1 nexthop-seg6local dum0 End_DX4 10.0.0.1 1

SRv6 routes are installed as NEXTHOP_IFINDEX routes because of seg6local
specification. seg6local routes depends the output device status instead
of routing-nexthop.

FYI:
In seg6local implementation, kernel don't care RTA_OIF on the nexthop
deeply but some requirement are exist as follow.
(a) DEV isn't loopback interface
(b) DEV's ipv6 status is enabled
(c) DEV's enslaving status is the same with target-route

Signed-off-by: Hiroki Shirokura <slank.dev@gmail.com>