fast system calls. They should now be fully comparable to
traditional system calls.

    Fix SEP stack allocation to look like a real thread stack.
    
    This fixes the "Trap on first sysenter instruction" problem,
    which can happen either as a result of an NMI or a breakpoint
    trap (TF bit set).
    
    Also make the user trampoline save and restore EFLAGS, so that
    the TF bit gets restored after a fast system call (it will have
    been cleared to avoid trap faults in the kernel).