-extend the pcu(9) API by a function which saves all context on the
current CPU, and use it if a CPU is taken offline
-add a bool argument to pcu_discard which tells whether the internal
"LWP has used the coprocessor" flag should be set or reset. The flag
is reported by pcu_used_p(). If set, future accesses should use the
state stored in the PCB. If reset, it should be reset to default.
The former case is useful for setmcontext().
With that, it should not be necessary anymore to manage the "FPU used"
state by an additional MD variable.

approved by matt

(drochner)