Fix broken build from 019632e12
Add userspace API for reaping processes
Remove need for internal reaper thread

The procmgr thread can serve this need, and it can be poked into
waking up and doing the work by the outgoing process sending an
asynchronous message that sneakily dual-purposes the
PROCMGR_MSG_TYPE_CHILD_FINISHED message. Normally, that message is
reserved for delivery to user processes who are being told that
their child has actually finished.