include: Fix level of indirection of parameters in GetNotifyCmdLine function of IBackgroundCopyJob2 interface in bits1_5.idl.

The parameters should be double pointers to strings, not pointers to strings.

Also remove incorrect [optional] attributes from parameters.

Fixes a bug in automatic redirects with content.

Spotted by Henri Verbeet.

Use SIZE_T as it matches the size that Windows programs expect whereas
size_t is platform-dependent.