[ADVAPI32] Additions for CreateProcessAsUserA/W().

[ADVAPI32] Additions for CreateProcessAsUserA/W().

- Use a common helper function used by both the ANSI and UNICODE functions
  so that the large code block that deals with setting the new process token
  becomes automatically common to both these functions, while the ANSI vs.
  UNICODE conversions are handled directly via the CreateProcessA/W() calls.

- Duplicate the token and acquire the process primary token assignment
  privilege before calling NtSetInformationProcess().