Add the MPIPE subsystem. This subsystem is used for 'pipelining' fixed-size

Add the MPIPE subsystem.  This subsystem is used for 'pipelining' fixed-size
allocations.  Pipelining is used to avoid lack-of-resource deadlocks by
still allowing resource allocations to 'block' by guarenteeing that an
already in-progress operation will soon free memory that will be immediately
used to satisfy the blocked resource.

Adjust the ATAold code to use the new mechanism and remove the code that
tried to back-off into PIO mode when resources were lacking.