The caller should be able to safely store the instance pointer before
his callback is invoked.

Allow implementations to install the "real" input later.

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

InputStream::Read() and InputStream::Seek() are called with the mutex
locked.  That means the implementation must not block, or unlock the
mutex before calling into blocking code.

Previously, a slow CD drive could stall the whole MPD process,
including the main thread, due to this problem.

Closes #149

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

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