dns: literals for any family, c-ares error text, link-local nameservers

[david-yu]

Part 3 of 10 of the IPv6 series, split out of #1 and rebased on current scylladb/seastar master. Based on #3, so the diff here is only this PR's 3 commits.

Why

Three more resolver gaps, which show up once an IPv6 nameserver is reachable:

• A numeric literal with a requested family went to c-ares, which cannot parse a zone (fe80::1%eth0 became a DNS query that failed with ARES_ENOTFOUND) and ignores the family for literals. The literal shortcut only ran for an unspecified family, the case b48ec904a made return whichever family answers first.
• The error category's hand-written copy of c-ares' table stopped at ARES_ECANCELLED, so ARES_ENOSERVER ("No DNS servers were configured") printed as "Unknown error". Those codes now get c-ares' own text; the texts already in the table stay, because applications and their tests match on them.
• aif_nametoindex/aif_indextoname were null, so c-ares dropped a link-local nameserver (nameserver fe80::1%eth0) whose zone it could not resolve.

What

• dns: resolve numeric literals without c-ares, for any requested family (fdcbe43b2)
• dns: fall back to c-ares' error text and wire the interface-name callbacks (23f154aa8)
• tests: cover the IPv6 side of the resolver (dbfe0849a) — AAAA answers, ip6.arpa PTR, TCP transport to [::1], literals per family; the mock nameserver now answers by QTYPE.

Behaviour change

	before	after
IPv4 literal resolved with family INET6	an AF_INET answer, ignoring the family	ARES_EBADFAMILY

Testing

test_error_category_texts pins the table's texts that applications match on (Not found, Connection refused) and checks that a code the table predates no longer renders as "Unknown error". An earlier version of this PR switched every code to ares_strerror; Redpanda's full ducktape run then failed four crash-loop tests whose log allow-lists expect C-Ares:4, unreachable_host.com: Not found.

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