Reduce header dependencies.

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

Decouple some more from GLib.

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

Replaces GLib's GError.

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

Don't use audio_format.reverse_endian.

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.

E.g., spotify isn't seekable.

Otherwise the song playing might go on forever.

Just there to support the "cdio_paranoia" input plugin.