Sets the UNIX file descriptors associated with this.
As a side-effect the [enum@Gio.DBusMessageHeaderField.NUM_UNIX_FDS] header field is set to the number of fds in fd_list (or
cleared if fd_list is `NULL`).
This method is only available on UNIX.
When designing D-Bus APIs that are intended to be interoperable, please note that non-GDBus implementations of D-Bus can usually only access file descriptors if they are referenced by a value of type `G_VARIANT_TYPE_HANDLE` in the body of the message.
| this |
a D-Bus message |
| fd_list |
A [class@Gio.UnixFDList], or `NULL` to clear |