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

Closes #169

It's not possible to set CURL options after curl_easy_perform(), and
thus the CURLOPT_RANGE had no effect.

From the CURLOPT_RANGE documentation: "The application does not have
to keep the string around after setting this option."

Fixes a GCC warning because %llu appears to be unsupported by the
Windows standard library.

Call SeekDone() to avoid the freeze bug.

This allows constructing an instance in any thread, and register it
inside the IOThread later.

The CurlRequest gives us a more low-level API to CURL without the
InputStream interface, integrated into our IOThread.

Simplifies the code and reduces the number of IOThread roundtrips.

default_value defaults to nullptr.

Fixes deadlock on small responses.

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

While seeking, metadata must not be updated.  ResponseBoundary() was
added in MPD 0.19.1, but I forgot to add the IsSeeking() check there.
This caused the "seekable" flag to reset.

Fixes playback of redirected streams.