This adds an extension point for TLS connections to gio, with a
gnutls-based implementation in glib-networking.
Full TLS support is still a work in progress; the current API is
missing some features, and parts of it may still be changed before
2.28.
https://bugzilla.gnome.org/show_bug.cgi?id=588189
There are apparently two incompatible ways of naming abstract sockets:
pad the sockaddr with 0s and use the entire thing as the name, or else
don't, and just pass a shorter length value to the relevant functions.
We previously only supported the former method. Add support for the
latter.
Also correctly handle "anonymous" unix sockaddrs (eg, the client side
of a connection, or a socketpair() socket), and add unix domain socket
support to the socket-client and socket-server test programs to make
sure this all works.
https://bugzilla.gnome.org/show_bug.cgi?id=615960
