Windows port: Fix improper access permissions to registry and DNS parsing from registry

Windows port: Fix improper access permissions to registry and DNS parsing from registry

- RegOpenKey() always try to open registry keys in full control mode, even if not needed.
  This could make Squid to fail when running as a non privileged user. RegOpenKeyEx() allow to
  specify only the needed priviledge and now is used instead.

- When parsing DNS setting into registry, a fixed size loop was used. Now the loop count is
  dynamic.