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.