Support interfaces with /31 netmasks (RFC 3021)

Hi,

While using pimd on a couple of routers, I noticed it ignored the router interconnect interfaces which have /31 netmasks as invalid. Actually RFC 3021 mandates that /31 IPv4 netmasks should be treated as valid and that they define a point-to-point interface without broadcast and subnet addresses.

I did a quick patch that treats /31 subnets as point-to-point links. I have tested it in production and it seems to work without breaking anything, but some further testing wouldn't harm.

Thanks,
Apollon