hw: generalize IPC-helping / help pager thread

[skalk]

Currently, on-demand paging in base-hw can conflict with the scheduling parameters one has chosen for a concrete scenario. Due to the fact that page-faults are propagated asynchronously via signals in between a faulting thread and the pager thread, and the fact that the pager thread itself runs without any priority nor CPU quantum. That means a high priority thread with CPU quantum in case of a page-fault gets deactivated, and potentially needs to wait for a long time until its fault gets resolved.
In contrast to ordinary IPC relations, there is no equivalent helping mechanism (donation of CPU time to callee) in the paging protocol. Any optimizations of the current scheduling algorithm like envisioned by issue #5117 will be still affected by this limitation. Thereby, first we should generalize the helping mechanism and apply it to page-fault resolution.