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.

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

Enforcing MADV_DONTFORK is a surprising limitation for this library
which aims to be generic.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.