Allow specifying local process reaper in procmgr

Currently, Redox follows the minimum allowed POSIX behavior when reparenting child process, by just reparenting them to init. But applications like contain might want this to be configurable, and Linux has prctl(PR_SET_CHILD_SUBREAPER, pid), so it would be interesting to similarly add a proc call etc. where this can be configured. An alternative for contain specifically could be to run a local process manager per sandbox, in which case specifying a custom init would be straightforward.