kernel: header tweaks

kernel: header tweaks

We add a new method to set p->p_udata on platform basis
We allow the page switch code to be passed a death argument so that flat and
similar models can do a one way copy if they are copying store to direct
mapped and the existing direct map is dead

(It's a common case when you fork and execve. The child has to put the parent
back but there's no point saving the child as it's going to free and realloc
a mapping)