HEADS UP - trapframe changes going into HEAD / Release

[Matthew Dillon]

    I really should have done this a long time ago but now there is
    pretty much no choice.

    The %gs segment register is being added to the trapframe.   A flags
    field is also being added (so userland can distinguish between read
    faults and write faults) when catching a segmentation fault signal),
    and the floating point context is being increased to 512 bytes
    to support SSE2.

    These changes effect all register frames, including the signal context,
    mcontext, and ucontext.

    Most applications do not use the register context made available in
    the signal frame, and will still work.  However, some may not after
    these changes and packages will almost certainly have to be updated
    for our release. 

    A full buildworld will also be required or gdb will be mightily
    confused about the register frame.

    If anyone knows of anything else that has to go into these frames
    while I'm breaking the structures, say so now!

						-Matt