fix not catching error on if_nametoindex

the method if_nametoindex returns 0 on error, check properly and print error.
Also print fount ifindex on -vv
send_rdisc: don't return -1 with stale errno

returning -1 without setting errno results in printing of misleading error messages.
Since ifindex should never be 0 anyway simply try sending the rdisc message and let
that fail correctly, which sets errno.