New release needed for QEMU IPv6 hostfwd support

Per request here: https://lists.nongnu.org/archive/html/qemu-devel/2021-05/msg02101.html filing this issue to discuss the next release. [I'm hoping I'm interpreting "create an issue there and discuss it" correctly.]

Alas I don't know what's involved in making a libslirp release (beyond simple mechanics).