As specified in RFC 2328 8.1: "On physical point-to-point networks,
the IP destination is always set to the address AllSPFRouters."

Note that this likely break setups with multiple neighbors on a network
configured as PtP, which worked before. These should be configured as
PtMP.

Thanks to Senthil Kumar Nagappan for the original patch and to Joakim
Tjernlund for suggestions.