Fix blackhole/reject routes.

Fix blackhole/reject routes.

Traditionally *BSD routing stack required to supply some
 interface data for blackhole/reject routes. This lead to
 varieties of hacks in routing daemons when inserting such routes.
With the recent routeing stack changes, gateway sockaddr without
 RTF_GATEWAY started to be treated differently, purely as link
 identifier.

This change broke net/bird, which installs blackhole routes with
 127.0.0.1 gateway without RTF_GATEWAY flags.

Fix this by automatically constructing necessary gateway data at
 rtsock level if RTF_REJECT/RTF_BLACKHOLE is set.

Reported by:	Marek Zarychta <zarychtam at plan-b.pwste.edu.pl>
Reviewed by:	donner
MFC after:	1 week