A /31-addressed interface requires a broadcast address of 255.255.255.255,
because there is no room for a proper broadcast address. Without this,
any packet destinated to the other end of the link is sent as broadcast,
which is incorrect.
For consistency with the Linux kernel, /32-addressed interfaces are
treated in the same way.
Signed-off-by: Baptiste Jonglez <git@bitsofnetworks.org>