netlink: fix addition of blackhole/reject routes.

• Make nhop_set_blackhole() set all necessary properties for the nexthop
• Make nexthops blackhole/reject based on the rtm_type netlink property instead of using rtflags.

Reported by: Marek Zarychta <zarychtam@plan-b.pwste.edu.pl>
MFC after: 3 days

(cherry picked from commit d2deebe21b591336fbd8915b37d409b25da54d4d)