Gets the UNIX file descriptors associated with this, if any.
This method is only available on UNIX.
The file descriptors normally correspond to `G_VARIANT_TYPE_HANDLE` values in the body of the message. For example, if [ method@GLib.Variant.get_handle] returns 5, that is intended to be a reference to the file descriptor that can be accessed by `g_unix_fd_list_get (list, 5, ...)`.
| this |
a D-Bus message |
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A [class@Gio.UnixFDList] or `NULL` if no file descriptors are associated |