If a kernel thread was created by a user land system call, the user

If a kernel thread was created by a user land system call, the user
land FPU context was saved to proc0.  This was an information leak
as proc0 is used to initialize the FPU at exec and signal handlers.
Never save the FPU to proc0, it has the initialization value.  Also
check whether the FPU has valid user land state that has to be
forked.
This bug is a regression from the eager FPU commit.  OK guenther@