ripd: allow to enable/disable the ECMP feature

ripd: allow to enable/disable the ECMP feature

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

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

* rip_zebra.c: adjust a debugging information, which shows the number
               of nexthops according to whether ECMP is enabled.
* ripd.c: rip_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, rip_ecmp_disable() is called to
          remove the multiple nexthops.
* ripd.h: Add a new member "ecmp" to "struct rip", 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: David Lamparter <equinox@opensourcerouting.org>
(cherry picked from commit 0b74a0a5db7bcf65bf68c44b547b02b1310b5cdb)