This adds a bit of overhead, but also adds flexibility to the API,
because arbitrary triggers may be invoked from that virtual method
implementation, not just Cond::signal().

The motivation for this is to make the handlers more dynamic, for the
upcoming buffering class utilizing ProxyInputStream.

To be used when a URL gets added.

Only _WIN32 is defined by the compiler, and WIN32 is not standardized
and may be missing.

Closes #169

Maybe this was once a required kludge for Windows, but it's not
anymore.

Eliminate dependency on io_thread_get().

The old struct config_param remains only for top-level string options.

Has been moved to class InputStream long ago.

Replace InputPlugin attributes.

Replaces the method Close().

Add enum InputResult which is a tri-state.  Input plugins may now fail
and just become unavailable.

Decouple some more from GLib.

Replaces GLib's GError.

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().

These fixes were mostly generated with `codespell' [0] and manually
reviewed.

[0] http://git.profusion.mobi/cgit.cgi/lucas/codespell/

Major API redesign: don't let the caller allocate the input_stream
object.  Let each input plugin allocate its own (derived/extended)
input_stream pointer.  The "data" attribute can now be removed, and
all input plugins simply cast the input_stream pointer to their own
structure (with an "struct input_stream base" as the first attribute).