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().

Work around a build failure due to windows.h macro.

We're going to use Vista features soon.  Sorry for those poor people
still on XP (or Windows, anyway).

This patch also adds extern "C" { } wrapper around string_util.h
to allow its usage in C++ code

Use the "url" attribute from the base class instead.

Don't use macro magic, don't dereference the block_param.

It is illegal to use it after going out of scope.

Switch WIN32 to this implementation to be able to use condition
variables, which is impossible with CriticalSection.

The main_cond variable was completely unnecessary.  The pc.cond object
can be used for both main->pc and pc->main.

Don't let the "client" pass its own GCond.  This was not used
consistently.

Fixes potential dead lock bug.

Use a bit field instead of a mutex-protected bool array.