See commit be137a19

Prepare to allow callers to pass custom request headers.

Allows the caller to decide whether to enable the feature.

The IcyMetaDataParser cannot be initialized already in OnHeaders(),
because it will be initialized late in that method; and there will not
be another OnHeaders() call, because streams with Icy metadata are not
seekable, thus there will not be another HTTP request.

This resolves the circular dependency between IcyInputStream and
CurlInputStream.

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.

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

Eliminate dependency on io_thread_get().

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.