Convenient method that behave differently than Read, and that will be used by
tag scanners.

This method will return in case of error, if the whole data is read or is EOF
is reached.

Seeking on NFS or SMB is cheap.  Actually, only HTTP streams are
expensive to seek.  This enables a few features on NFS/SMB files, for
example Ogg tags.

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

Allow compiling test programs with only selected plugins.

Replace InputPlugin attributes.

Replaces the method Close().

Merge some duplicate code.

Decouple some more from GLib.

Replaces GLib's GError.

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

Move code from the Vorbis decoder plugin.

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.

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

All close() implementations must call this method.