[krbdev.mit.edu #2659] rpc code has no IPv6 support

[Ken Raeburn via RT]

We install svc.h, which has sockaddr_in structures for the local and
remote addresses in SVCXPRT and directly accessed with svc_getcaller. 
So any change to support IPv6 will break binary compatibility with our
currently deployed RPC libraries, necessitating a major version number
bump for the library.

Once we figure out what to do about IPv6 in the RPC code, see about
integrating the IPv6 bits of Denis Vlasenko's patch to the RPC library
(for IPV6_PKTINFO) in 3306.