GUnixSocketAddress: fix construct parameter issue

GUnixSocketAddress: fix construct parameter issue

GUnixSocketAddress has some very strange logic for interpreting its
construct paramters.  This logic behaves differently in these two cases:

  g_object_new (G_TYPE_UNIX_SOCKET_ADDRESS,
                "abstract", FALSE,
                "address-type", ...,
                NULL);

and

  g_object_new (G_TYPE_UNIX_SOCKET_ADDRESS,
                "address-type", ...,
                NULL);

even though the default value for "abstract" is already FALSE.

Change the way the code works so that it is not sensitive to people
merely setting a property to its default value.

https://bugzilla.gnome.org/show_bug.cgi?id=698686