Make getaddrinfo an asynchronous lookup with promise and callback variants

[guybedford]

emscripten_dns_lookup(node, service, hints) is the getaddrinfo body as one __async: ['promise', 'callback'] function: it resolves to a newly allocated addrinfo list (or a negative EAI_* code), synchronously for numeric addresses and errors, and under NODERAWSOCKETS after a node:dns lookup for a hostname, or EAI_AGAIN where sync says the caller cannot wait. getaddrinfo() itself becomes a C wrapper over it; its hand-written ASYNCIFY/pthread dispatch goes away.

emscripten_dns_lookup_promise() and emscripten_dns_lookup_callback() are its _promise and _callback variants, so a real DNS lookup can be awaited without blocking from any stack: as an em_promise_t, or through callback(err, result, arg) on the calling thread (err is never set: a failed lookup is an EAI_* result). All three are declared in emscripten.h, so gen_sig_info generates their signatures as usual. Supersedes #27742.

intptr_t emscripten_dns_lookup(const char *name, const char *service, const struct addrinfo *hints);
em_promise_t emscripten_dns_lookup_promise(const char *name, const char *service, const struct addrinfo *hints);
void emscripten_dns_lookup_callback(const char *name, const char *service, const struct addrinfo *hints,
                                    void (*callback)(bool err, intptr_t result, void *arg), void *arg);

Tests: other.test_dns_lookup_async, sockets_node.test_noderawsockets_dns_async (with PROXY_TO_PTHREAD); existing getaddrinfo tests unchanged.

Made with AI assistance under my review