Support muliple RAs on single interface

Support muliple RAs on single interface

IETF is moving toward implementing IPv6 multihoming by sending
multiple RAs on a single interface:
- draft-ietf-intarea-provisioning-domains-00
- draft-ietf-rtgwg-enterprise-pa-multihoming-02

odhcpd supports configuration of multiple software interfaces
on the same physical interface, which already advertises
multiple RAs, but had two issues:
- Each RA includes all the prefixes available on the interface.
- Replies to sollicits with a single RA.

This patch introduces the prefix_filter configuration parameter
which allows filtering prefixes that are sent in a given RA,
and fixes the sollicit code in order to reply with all the RAs
that are configured on a given interface.

Signed-off-by: Pierre Pfister <ppfister@cisco.com>