Users can enable supervise-daemon simply by declaring
`supervisor=supervise-daemon` in /etc/conf.d/radiusd.

See my mail from Tue, 15 Dec 2020 15:33:16 +0100 in alpine-devel
mailing-list in topic "Use of supervise-daemon in Alpine".
(I cannot simply reference it from ML archive because it's broken.)

This partially reverts 1a33da02.