In the copy(9) function, make sure to remember curproc accross the bcopy()

In the copy(9) function, make sure to remember curproc accross the bcopy()
call, instead of &curproc. The copy routine may sleep and we may resume on
a different processor. This has been plaguing the alpha MULTIPROCESSOR kernels
since the very beginning; it's amazing this did not cause more havoc.

Joint debugging and hair pulling with dlg@ and deraadt@; ok dlg@