If we have an input parameter (or return value) we need to use (nullable).
However, if it is an (inout) or (out) parameter, (optional) is sufficient.
It looks like (nullable) could be used for everything according to the
Annotation documentation, but (optional) is more specific.
Add a new GDtlsConnection interface, plus derived GDtlsClientConnection
and GDtlsServerConnection interfaces, for implementing Datagram TLS
support in glib-networking.
A GDtlsConnection is a GDatagramBased, so may be used as a normal
datagram socket, wrapping all datagrams from a base GDatagramBased in
DTLS segments.
Test cases are included in the implementation in glib-networking.
https://bugzilla.gnome.org/show_bug.cgi?id=752240
