Remove the "whence" parameter that is not actually necessary, and only
complicates the InputStream implementations.

Caused by commit 0b4fa41a

Replace InputPlugin attributes.

Replaces the method Close().

Prepare for adding virtual methods.

Decouple some more from GLib.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Replaces GLib's GError.

Eliminate input_stream_init() and input_stream_deinit().

Hide the definition from C code, to prepare the transition to C++.

Add GMutex, GCond attributes which will be used by callers to
conditionally wait on the stream.

Remove the (now-useless) plugin method buffer(), wait on GCond
instead.  Lock the input_stream before each method call.  Do the same
with the playlist plugins.

To check for errors without reading.  The decoder thread wants to do
that, before it passes the input stream to the plugin.

Reduce heap usage by reducing the number of malloc() / free() calls.

Update the struct attributes, important for facades like the "rewind"
plugin.  To replace buffer().

The assertion added in MPD 0.15.14 was too much, it failed when the
MIME type of a stream was NULL.