[Patch] Allow to disable root privileges with CYGWIN=noroot

[Christian Franke]

Corinna Vinschen wrote:
> ...and maybe it's time to create a cygwin_internal call which replaces
> cygwin_set_impersonation_token and deprecate cygwin_set_impersonation_token
> in the long run.  So, instead of the above we could have this call
> taking a HANDLE and a BOOL value:
>
>   cygwin_internal (CW_SET_EXTERNAL_TOKEN, token_handle, restricted?);
>
>   

OK.

I have a very first experimental version which works for me. It also 
requires a new flag 'cygheap->user.is_restricted_token' to tell 
spawn_guts() to use CreateProcessAsUser().

I will post the patch in a few days.

A question:

Why does seteuid32() call 'set_cygwin_privileges ()' on 'curr_imp_token' 
and not on 'curr_primary_token' ? The curr_primary_token is used for 
impersonation and therefore the privileges are not set for the thread 
itself.

Christian