input/rewind: new input_stream wrapper to allow stream rewinding

This replaces the rewinding buffer code from the CURL input plugin.
It is more generic, and allows rewinding even when the server sends
Icy-Metadata (which would have been too difficult to implement within
the CURL plugin).

This is a rather complex patch for the stable branch (v0.15.x), but it
fixes a serious problem: the "vorbis" decoder plugin was unable to
play streams with Icy-Metadata, because it couldn't rewind the stream
after detecting the codec (Vorbis vs. FLAC).

src/input/curl_input_plugin.c

@@ -732,6 +732,18 @@ input_curl_easy_init(struct input_stream *is)
 	return true;
 }
 
+void
+input_curl_reinit(struct input_stream *is)
+{
+	struct input_curl *c = is->data;
+
+	assert(is->plugin == &input_plugin_curl);
+	assert(c->easy != NULL);
+
+	curl_easy_setopt(c->easy, CURLOPT_WRITEHEADER, is);
+	curl_easy_setopt(c->easy, CURLOPT_WRITEDATA, is);
+}
+
 static bool
 input_curl_send_request(struct input_curl *c)
 {