Create new in6_purgeifaddr() which purges bound ifa prefix if it gets unused.

[melifaro]

Summary

Currently if_purgeifaddrs() uses in6_purgeaddr() to remove all
ifaddrs for IPv6. The problem is that in6_purgeaddr() is a
low-level function used in the other parts of inet6 code to
handle exactly ifa removal.
Create a higher-level wrapper which handles unused prefix usecase
and use it in if_purgeifaddrs().