r4147: converted from NT_USER_TOKEN to struct security_token

r4147: converted from NT_USER_TOKEN to struct security_token

this is mostly just a tidyup, but also adds the privilege_mask, which
I will be using shortly in ACL checking.

note that I had to move the definition of struct security_token out of
security.idl as pidl doesn't yet handle arrays of pointers, and the
usual workaround (to use a intermediate structure) would make things
too cumbersome for this structure, especially given we never encode it
to NDR.