Avoid AI_ADDRCONFIG netlink path for DNS lookups (#560, #524)

[bsergean]

Fixes #560
Fixes #524

Problem

DNSLookup::getAddrInfo always passed AI_ADDRCONFIG to getaddrinfo:

hints.ai_flags = AI_ADDRCONFIG | AI_NUMERICSERV;

That flag causes two distinct failures:

• Crash on DNS lookup #560 (crash): on glibc, AI_ADDRCONFIG enumerates the configured interfaces through a netlink socket (__check_pf). Under heavy threaded churn that path can abort the whole process from inside getaddrinfo (__netlink_assert_response → SIGABRT). The reported reproducer connects to 127.0.0.1 — a numeric literal that needs no name resolution at all.
• Localhost DNS lookup fails without network connection #524 (offline localhost): AI_ADDRCONFIG only returns addresses for families with a configured non-loopback interface. With no network (cable unplugged / no wifi), even localhost fails to resolve.

Fix

1. Numeric IP literals (detected with the library's inet_pton) now use AI_NUMERICHOST and drop AI_ADDRCONFIG. There's no name to resolve, so glibc skips the netlink enumeration entirely — directly avoiding the Crash on DNS lookup #560 abort for IP hosts, and making literal lookups faster. This covers the exact 127.0.0.1 reproducer.
2. Real hostnames keep AI_ADDRCONFIG for its intended benefit, but if the first getaddrinfo fails we retry once without it, so loopback-resolving names still work offline (Localhost DNS lookup fails without network connection #524). No new API/config knob required.

Testing

• Builds clean (-DUSE_TLS=ON -DUSE_OPEN_SSL=ON, CMAKE_CXX_STANDARD 11).
• Smoke test via DNSLookup::resolve: 127.0.0.1, ::1, 8.8.8.8 (numeric path) and localhost (AI_ADDRCONFIG path) all resolve correctly.

Notes / scope

• This is the cleanest mitigation for the Crash on DNS lookup #560 reproducer (numeric host → no netlink). For non-numeric hostnames under extreme threaded churn the glibc netlink abort is a deeper libc-level issue; if that surfaces in practice, a follow-up could drop AI_ADDRCONFIG more broadly, but that has IPv4/IPv6 selection trade-offs so I kept this change conservative.
• Double EBADF Error Callback On DNS Fail #565 (double callback on DNS fail) is a separate auto-reconnection behavior, not addressed here.

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