LookupServer: Minor overhaul

- Break out request building into a DNSRequest class.
- Break out response parsing into a DNSResponse class.

A DNSRequest contains one or more DNSQuestion objects.
A DNSResponse contains all the DNSQuestions asked, and a DNSAnswer
object for each answer.

Servers/LookupServer/DNSRequest.cpp

@@ -0,0 +1,49 @@
+#include "DNSRequest.h"
+#include "DNSPacket.h"
+#include <AK/BufferStream.h>
+#include <arpa/inet.h>
+
+#define C_IN 1
+
+static u16 s_next_id = 0;
+
+DNSRequest::DNSRequest()
+    : m_id(s_next_id++)
+{
+}
+
+void DNSRequest::add_question(const String& name, u16 record_type)
+{
+    ASSERT(m_questions.size() <= UINT16_MAX);
+    m_questions.empend(name, record_type, C_IN);
+}
+
+ByteBuffer DNSRequest::to_byte_buffer() const
+{
+    DNSPacket request_header;
+    request_header.set_id(m_id);
+    request_header.set_is_query();
+    request_header.set_opcode(0);
+    request_header.set_truncated(false);
+    request_header.set_recursion_desired(true);
+    request_header.set_question_count(m_questions.size());
+
+    auto buffer = ByteBuffer::create_uninitialized(m_questions.size() * 4096);
+    BufferStream stream(buffer);
+
+    stream << ByteBuffer::wrap(&request_header, sizeof(request_header));
+
+    for (auto& question : m_questions) {
+        auto parts = question.name().split('.');
+        for (auto& part : parts) {
+            stream << (u8)part.length();
+            stream << part;
+        }
+        stream << '\0';
+        stream << htons(question.record_type());
+        stream << htons(question.class_code());
+    }
+    stream.snip();
+
+    return buffer;
+}