Abstracted open_registry().

Abstracted open_registry().

Added service_registry_path() to construct a registry path for a service
which may contain the Parameters subkey and optional sub-subkey.  If no
Parameters subkey is needed the constructed path references the native
service parameters.

The open_registry() function is now a wrapper around the new
open_registry_key() function which opens an arbitrary key.

The new open_service_resgistry() function also wraps open_registry_key()
to access native parameters.