This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Has been moved to class InputStream long ago.

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

Replace InputPlugin attributes.

Replaces the method Close().

Prepare for adding virtual methods.

Decouple some more from GLib.

Replaces GLib's GError.

Add the interface ArchiveVisitor.

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.