Re-factor into separate compression module

Introduce a `conn_interface` to simplify the decision logic which API
we must call.

This also fixes some bugs of the previous commit.

Signed-off-by: Steffen Jaeckel <jaeckel-floss@eyet-services.de>

src/compression_dummy.c

@@ -0,0 +1,36 @@
+/* SPDX-License-Identifier: MIT OR GPL-3.0-only */
+/* compression_dummy.c
+** strophe XMPP client library -- Dummy Compression
+**
+** Copyright (C) 2024 Steffen Jaeckel <jaeckel-floss@eyet-services.de>
+**
+**  This software is provided AS-IS with no warranty, either express
+**  or implied.
+**
+**  This program is dual licensed under the MIT or GPLv3 licenses.
+*/
+
+/** @file
+ *  Dummy Compression.
+ */
+#include <strings.h>
+#include <errno.h>
+
+#include "common.h"
+
+int compression_init(xmpp_conn_t *conn)
+{
+    conn->compression.supported = 0;
+    return -1;
+}
+
+void compression_free(xmpp_conn_t *conn)
+{
+    UNUSED(conn);
+}
+
+void compression_handle_feature_children(xmpp_conn_t *conn, const char *text)
+{
+    UNUSED(text);
+    conn->compression.supported = 0;
+}