Bind to dual-stack socket by default [IPv6]

[margau]

I've been trying to set up the notify_push daemon using a reverse proxy on another machine. The internal network is IPv6 only (as we're living in 2021). Unfortunately, the daemon does not start an IPv6 Socket on ::, but only a Legacy Socket:

tcp 0 0 0.0.0.0:7867 0.0.0.0:* LISTEN 8246/notify_push

I'd suggest using a Socket bound to :: with the IPV6_V6ONLY socket option deactivated. That way, both IPv6 and Legacy will be supported in a single socket.

As a workaround, you can set "Environment=BIND=::" to force listening on IPv6. 9f67831 as given in #47 does not fix this, as it does not work when the reverse proxy does communicate only over IPv6.