server: implement passing a process security descriptor to CreateProcess.

For now the function "NTDLL_create_struct_sd" has been duplicated in kernel32. This is needed because kernel32 makes the server call.
Kernel32 currently makes the server call because NtCreateProcess(Ex) has not been implemented in ntdll. When NtCreateProcessEx (and NtCreateThreadEx) gets implemented,
the server call will be made from within ntdll instead, and this extra function in kernel32 will no longer be needed.