ipv6: introduce union iaddr

ipv6: introduce union iaddr

Replace many occurrences of sockaddr_in by a new union, iaddr.

Everywhere that fills in an address has been modified to look into the
subfields of iaddr.  But there is not yet any support for a union
iaddr to contain anything other than a sockaddr_in.  This will be
added gradually in forthcoming patches, starting at consumers and
working back.

Additionally, a couple of places that specified a port and address as
a uint16_t and uint32_t have been converted.

We have changed only transport addresses - that is, addresses on the
public network.  VPN addresses remain IPv4 only.

We provide a few helper functions for manipulating union iaddr, such
as iaddr_to_string (which replaces saddr_to_string).

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>