lib, zebra: Add ability to read kernel notice of TRAP/OFFLOAD

lib, zebra: Add ability to read kernel notice of TRAP/OFFLOAD

The linux kernel is getting RTM_F_TRAP and RTM_F_OFFLOAD for
kernel routes that have an underlying asic offload.  Write the
code to receive these notifications from the linux kernel and
to store that data for display about the routes.

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