zebra: Add ability to dump routes received from fpm_listener

zebra: Add ability to dump routes received from fpm_listener

The fpm_listener currently has no ability to store the list
of prefixes that it has received.  Modify the code to store
the prefixes in a typesafe RB Tree.  Additionally modify
the code such that when a SIGUSR1 is received to dump
the routes out.  If the operator specifies a -z <filename>
then write the routes to that file.  It will overwrite
the last version of the file written.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>