Fix undirected broadcast sends for the case where SO_DONTROUTE has also
been set at the socket layer, in our somewhat convoluted IPv4 source
selection logic in ip_output().
IP_ONESBCAST is actually a special case of SO_DONTROUTE, as 255.255.255.255
must always be delivered on a local link with a TTL of 1.
If IP_ONESBCAST has been set at the socket layer, also perform destination
interface lookup for point-to-point interfaces based on the destination
address of the link; previously it was not possible to use the option with
such interfaces; also, the destination/broadcast address fields map to the
same field within struct ifnet, which doesn't help matters.
Reviewed by: andre