This eliminates some complexity from class CurlRequest.

Allows constructing the request first and set the URL later.  This is
needed because curl_easy_escape() is needed to construct the URL,
which however needs the CURL "easy" handle created by the Request class.

Bugs found by Coverity.

libcurl's WRITEFUNCTION is pretty fragile; if we destroy the CURL*
instance or even unregister it using curl_multi_remove_handle(),
libcurl will crash instantly.  But still we need to be able to handle
exceptions from inside the WRITEFUNCTION, and call
CurlResponseHandler::OnError(), which may destroy the whole thing.  As
a workaround, I use DeferredMonitor to postpone the OnError() call
into a stack frame which is allowed to destroy the request.

Let the caller decide what to do with the exception.

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.

Allows forward-declaration.

Use CRITICAL_SECTION and CONDITION_VARIABLE.  This requires Windows
Vista or newer.  It fixes problems with GLib threading objects that
were implicitly created by static constructors before g_thread_init().

Don't call g_error(), which will abort the process and dump core.

This updates the copyright header to all be the same, which is
pretty much an update of where to mail request for a copy of the GPL
and the years of the MPD project. This also puts all committers under
'The Music Player Project' umbrella. These entries should go
individually in the AUTHORS file, for consistancy.

Renamed functions.

This is a rather huge patch, which unfortunately cannot be splitted.

Instead of using our custom ioops.h library, convert everything to use
the GLib main loop.

Merged freeAllListenSockets() into closeAllListenSockets(), because
this is its only caller.

"LOG_H" is a macro which is also used by ffmpeg/log.h.  This is
ffmpeg's fault, because short macros should be reserved for
applications, but since it's always a good idea to choose prefixed
macro names, even for applications, we are going to do that in MPD.