ripngd: allow to enable/disable the ECMP feature

ripngd: allow to enable/disable the ECMP feature

Introduce a new command "[no] allow-ecmp" to enable/disable the
ECMP feature in RIPng. By default, ECMP is not allowed.

Once ECMP is disabled, only one route entry can exist in the list.

* ripng_zebra.c: adjust a debugging information, which shows the number
                 of nexthops according to whether ECMP is enabled.
* ripngd.c: ripng_ecmp_add() will reject the new route if ECMP is not
            allowed and some entry already exists.
            A new configurable command "allow-ecmp" is added to control
            whether ECMP is allowed.
            When ECMP is disabled, ripng_ecmp_disable() is called to
            remove the multiple nexthops.
* ripngd.h: Add a new member "ecmp" to "struct ripng", indicating whether
            ECMP is allowed or not.

Signed-off-by: Feng Lu <lu.feng@6wind.com>
Reviewed-by: Alain Ritoux <alain.ritoux@6wind.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Acked-by: Vincent Jardin <vincent.jardin@6wind.com>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
(cherry picked from commit 72855b16b72e9ad2c7eb0c0bfd8f5985f779608f)