Use IFA_F_NOPREFIXROUTE to avoid configuring a /128 address

[AGWA]

rdiscd currently configures addresses with a /128 prefix length rather than the prefix length advertised by the router (e.g. /64). Since rdiscd also explicitly adds a route to the prefix, the /128 address should be functionally equivalent to an address with the advertised prefix length. Nevertheless, this is unexpected and may cause confusion as well as functional problems that have not yet been discovered.

The reason why rdiscd does this is because the kernel automatically adds a route to the prefix if you configure an address with a prefix length besides /128, but rdiscd needs to be in charge of adding that route. Recent versions of the kernel (3.14+) have a flag, IFA_F_NOPREFIXROUTE, to prevent the automatic addition of the prefix route, and recent versions of iproute2 have an option to ip-address, noprefixroute, to use this flag.

rdiscd should use this flag if the kernel and iproute2 provide it, and then configure the address with the true prefix length instead of /128.