struct sin6_addr has two additional fields that struct sin_addr
doesn't. Add support for those to GInetSocketAddress, and make sure
they don't get lost when converting between glib and native types.
https://bugzilla.gnome.org/show_bug.cgi?id=635554
This is nice for some callers so they can report an error.
It is also required to support opional address types like
abstract paths for unix domain sockets.
Types and methods for dealing with IPv4 and IPv6 addresses (and UNIX
domain socket addresses under UNIX). This does not include code for
actual socket I/O.
Originally from "gnio". Much of the code was written by Christian
Kellner, Samuel Cormier-Iijima, and Ryan Lortie.
Part of #548466.
