fpm: send all next hop groups on startup

fpm: send all next hop groups on startup

Implement the next hop group send on startup if you are using
them. Normally you will only have them if you are already using this
Linux kernel feature.

NOTE: to make sure all next hop groups exist, we send/enqueue all next
hop groups first and then we send routes. The RIB route walk start is
at the end of the function `fpm_nhg_send()`.

Signed-off-by: Rafael Zalamena <rzalamena@opensourcerouting.org>