Fix a failed test from ntdll:NtSystemInformation in Windows 2003

The following test was failing in Windows Server 2003:

/* Test without privilege */
NtSetSystemInformation(SystemTimeAdjustmentInformation, &SetTimeInfo, sizeof(SetTimeInfo));
ok_ntstatus(Status, STATUS_PRIVILEGE_NOT_HELD);

The reason is that when the call to the NT function is performed, the process token has the SE_PRIVILEGE_ENABLED already set thanks to the GetPrivilege() previous call.

The Patch attached:

• Implements ResetPrivilege() which resets the SE_PRIVILEGE_ENABLED before performing the call.
• Fixes a token handle leak.