This way, IsFileNotFound() can detect status 404.

See commit be137a19

For remote files (not streams), this downloads as quickly as possible
to a large buffer instead of throttling the stream during playback.
Throttling can make the server impatient and it may then disconnect.

This is what Qobuz and Tidal do, and this commit attempts to solve
this by not letting the Qobuz/Tidal server wait (closes #241).

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.

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().