Device layering work patch #1

[Matthew Dillon]

    Well, there are plenty of things I throw into the tree before its time,
    usually to my woe (like the FP copy code).  

    I'm not taking any chances with the device layering code.  One mistake
    and it could start overwriting one disk partition when you were trying
    to write to another, for example.  Unlike the FP code, which I thought
    I had working when I committed it, I knew for a fact before patch #5
    that the device layering had serious issues that needed to be worked out.

    I do think the patch is finally getting to the point where it can be 
    committed, thanks to excellent comments from the gallary.

						-Matt