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.h

@@ -20,6 +20,16 @@
 #ifndef MPD_INPUT_CURL_H
 #define MPD_INPUT_CURL_H
 
+struct input_stream;
+
 extern const struct input_plugin input_plugin_curl;
 
+/**
+ * This is a workaround for an input_stream API deficiency; after
+ * exchanging the input_stream pointer in input_rewind_open(), this
+ * function is called to reinitialize CURL's data pointers.
+ */
+void
+input_curl_reinit(struct input_stream *is);
+
 #endif