Introduce IPv6-enabled version of get_client_address. The legacy mount

command could use this eventually as well.

If this new function fails to discover an appropriate callback address, it
fills in an ANY address to indicate to the server that it should not call the
client back (ie delegations are disabled in this case).

The user can specify a callback address via the clientaddr= mount option in
this case to enable delegation.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Steve Dickson <steved@redhat.com>