Sets the UNIX file descriptors associated with this.
As a side-effect the g_dbus_message_header_field_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.
A UnixFDList or null.