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_stream.c

@@ -22,6 +22,7 @@
 #include "conf.h"
 
 #include "input/file_input_plugin.h"
+#include "input/rewind_input_plugin.h"
 
 #ifdef ENABLE_ARCHIVE
 #include "input/archive_input_plugin.h"
@@ -131,6 +132,8 @@ input_stream_open(struct input_stream *is, const char *url)
 			assert(is->plugin->eof != NULL);
 			assert(!is->seekable || is->plugin->seek != NULL);
 
+			input_rewind_open(is);
+
 			return true;
 		}
 	}