Read curproc from cpu structure, but be careful because after a schedule event

Read curproc from cpu structure, but be careful because after a schedule event
myproc() points to a different thread.

   myproc();
   sched();
   myproc();  // this proc maybe different than the one before sched

Thus, in a function that operates on one thread better to retrieve the
current process once at the start of the function.