Allow autobound UDP sockets to send to IPv4 and IPv6

Allow autobound UDP sockets to send to IPv4 and IPv6

When writeDatagram is called without first binding the UDP socket,
then bind it as QHostAddress::Any.
This allows the same socket to be used to sent to both IPv4 and
IPv6 destination addresses.

Allowing the OS to autobind the socket inside sendTo() may
result in a single protocol socket.

Task-number: QTBUG-5275
Change-Id: I2b76507e8a8a38369c6eafb61ce4191d1d6cc930
Reviewed-by: Richard J. Moore <rich@kde.org>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Martin Petersson <Martin.Petersson@nokia.com>