Switch net.add_addr_allfibs default to 0.

The goal of the fib support is to provide multiple independent
routing tables, isolated from each other.
net.add_addr_allfibs default tries to shift gears in the opposite
direction, unconditionally inserting all addresses to all of the fibs.

There are use cases when this is necessary, however this is not a
default expected behaviour, especially compared to other implementations.

Provide WARNING message for the setups with multiple fibs to notify
potential users of the feature.

Differential Revision: https://reviews.freebsd.org/D26076