net: listen on an unspecified address binds any family

[david-yu]

Part 9 of 10 of the IPv6 series, split out of #1 and rebased on current scylladb/seastar master. Based on #9, so the diff here is only this PR's commit.

Why

A default-constructed socket_address has been AF_UNSPEC since 453d531b0 (scylladb/seastar#694), but listen() still turned it into 0.0.0.0, so listen(socket_address{}) ("any address, any port") was unreachable on an IPv6-only host.

What

• net: listen on an unspecified address binds any family, not just IPv4 (20cade105) — binds [::] with IPV6_V6ONLY off where the stack has IPv6, so IPv4 peers still arrive, and 0.0.0.0 where it does not, both through inet_address(family) (669685c78) as the IPv4 branch always did. An explicitly set listen_options::ipv6_only is kept.

Behaviour change

On a host with IPv6, listen(socket_address{}) binds [::] instead of 0.0.0.0. IPv4 clients still connect; the socket reports them as ::ffff:a.b.c.d, and http_server/rpc::server unmap them (PR 4).

Testing

Listening on socket_address{} and connecting from 127.0.0.1: the listener is AF_INET6 on the any-address, and the peer reads as IPv4 after unmapping.

On the core-aws VM (Ubuntu 24.04, GCC 14, C++23, RelWithDebInfo, GnuTLS and OpenSSL in one tree), this PR's head builds and passes dns, ipv6, socket, rpc, httpd, httpd_openssl, tls, tls_openssl, network_interface, websocket and unix_domain with SEASTAR_TEST_REQUIRE_IPV6=1, so none of their IPv6 cases skipped.

Stack

1. tests, ci: fail instead of skip when IPv6 is expected #2 tests, ci: fail instead of skip when IPv6 is expected
2. dns: reach nameservers over IPv6 #3 dns: reach nameservers over IPv6
3. dns: literals for any family, c-ares error text, link-local nameservers #4 dns: literals for any family, c-ares error text, link-local nameservers
4. net, http, rpc: dual-stack listeners (IPV6_V6ONLY, IPv4-mapped peers) #5 net, http, rpc: dual-stack listeners (IPV6_V6ONLY, IPv4-mapped peers)
5. net: IPv6 zones and strict literal parsing #6 net: IPv6 zones and strict literal parsing
6. tls: do not send IP literals in the SNI extension #7 tls: do not send IP literals in the SNI extension
7. tls: verify server_name on OpenSSL, opt-in #8 tls: verify server_name on OpenSSL, opt-in
8. net: ipv6_addr equality and hash, ipv4_addr(socket_address) throws instead of terminating #9 net: ipv6_addr equality and hash, ipv4_addr(socket_address) throws instead of terminating
9. net: listen on an unspecified address binds any family #10 net: listen on an unspecified address binds any family ← this PR
10. http: build the Host header from an address in authority form #11 http: build the Host header from an address in authority form