ExitPolicy torrc dual-stack (unify accept/accept6 and reject/reject6)

Mentioned in #21524 (closed) I suggested that there is not much use to distinguish accept/accept6 and reject/reject6 in the torrc.

In most cases, it is unambiguous what we want, e.g.:

• 1.2.3.4 we obviously want IPv4;
• [2000::]/8 we obviously want IPv6;
• * and private we are probably fine with dualstack;
• and if we don't want dualstack, we have 0.0.0.0/0 for IPv4 wildcard and [::]/0 for IPv6. Specific to #21524 (closed) we could have private4 and private6 in addition to dualstack private.

In all the above cases, we don't really need distinguishable accept4 and accept6 keywords, we can deduce the address family straight from the rules themselves. So, this proposal would modify the accept and reject keywords to be dual-stack.

This of course is a big proposal and will mess up current torrcs everywhere, so we need to figure out:

• Are we interested in such a change?
• If so, how do we wean off people from accept6 and reject6?
• Also, how do we handle users' expectations that accept *:* is IPv4-only currently? Are we okay to suddenly make that dual-stack?