- Fix discrepancy between headers and implementation of
  I_RpcBindingSetAsync when compiling under WINNT.
- Eliminate race conditions in the relationship between "listen_count"
  and "std_listen" in rpc_server.h.
- Code-stubs for RpcServerUnregisterIf, RpcServerUnregisterIfEx.
- Attempt to implement RpcMgmtStopServerListening (definitely not
  happy just yet).
- RpcServerUnregisterIfEx wasn't xp-sepcific after all; this leads me to
  suspect there are more such mistakes in the spec-file comments, but I
  haven't looked into it or fixed them yet (coming soon).
- Cleanups, trace-enhancements, etc.