pimd: Fix connected route nexthop fix from 66f5152f

pimd: Fix connected route nexthop fix from 66f5152f

Fix a couple of problems in my 1st fix for PIM nexthops reachable via a
connected route:

Use NEXTHOP_TYPE_IPV4_IFINDEX instead of NEXTHOP_TYPE_IPV4 since we add an
IPv4 address to an already known ifindex.

Assign nexthop_tab[num_ifindex].protocol_distance and .route_metric before
incrementing num_ifindex.

Revert the default: to individual switch case statement conversion in
zclient_read_nexthop() as requested by donaldsharp in #2347

Signed-off-by: Martin Buck <mb-tmp-tvguho.pbz@gromit.dyndns.org>