net, http, rpc: dual-stack listeners (IPV6_V6ONLY, IPv4-mapped peers)

[david-yu]

Part 4 of 10 of the IPv6 series, split out of #1 and rebased on current scylladb/seastar master. Based on #4, so the diff here is only this PR's 2 commits.

Why

On a dual-stack host a [::] listener's IPv4 clients arrive as ::ffff:a.b.c.d, which does not compare, hash or test as loopback like a.b.c.d. http_server and rpc::server handed that form to handlers and filter_connection, so address-keyed policies written for IPv4 stopped matching once a listener moved to [::]. Whether a [::] listener accepts IPv4 at all also followed the host's net.ipv6.bindv6only, with no way to ask.

What

• net: let listen_options pin IPV6_V6ONLY (372fb6ffd) — listen_options::ipv6_only (std::optional<bool>; unset keeps today's behaviour).
• net, http, rpc: treat IPv4-mapped IPv6 peers as IPv4 (a788a963a) — inet_address::is_ipv4_mapped()/unmapped() and socket_address::unmapped(); http_server and rpc::server record unmapped peer and local addresses.

Builds on

inet_address's in_addr conversion already unwrapped the mapped form; unmapped() is that logic, made reusable. Sockets keep reporting what the kernel returns, as datagram channels do since 731ac075f (scylladb/seastar#3700); only the two servers that hand the address to application code normalise it.

Testing

A [::]:0 listener reached from 127.0.0.1 with ipv6_only false (accepted, peer ::ffff:127.0.0.1) and true (ECONNREFUSED); an http_server on [::] reports its IPv4 client as 127.0.0.1. unmapped() is the identity for anything that is not ::ffff:a.b.c.d.

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) ← this PR
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
10. http: build the Host header from an address in authority form #11 http: build the Host header from an address in authority form