zebra: Accept and process RAs with lifetime of zero

zebra: Accept and process RAs with lifetime of zero

Fix code to not discard received RAs with a lifetime of 0. The router lifetime
is only applicable for default router processing which is not relevant here.
For the purposes of BGP unnumbered, the neighbor should be learnt without
consideration of the value of router lifetime in received RA.

Note: This patch brings in a portion of the earlier commit
690baa53592320dddee5c729f959150cc9a72699 - this included some additional
changes which have been reverted.

Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>
Reviewed-by:   Daniel Walton <dwalton@cumulusnetworks.com>
Ticket: CM-10943
Reviewed By: CCR-4611
Testing Done: bgp-smoke