scheduler_start: move static timeout structs into callback functions

scheduler_start: move static timeout structs into callback functions

Move the schedcpu() and update_loadavg() timeout structs from
scheduler_start() into their respective callback functions and
statically initialize them with TIMEOUT_INITIALIZER(9).

The structs are already hidden from the global namespace and the
timeouts are already self-managing, so we may as well fully
consolidate things.

Thread: https://marc.info/?l=openbsd-tech&m=169488184019047&w=2

"Sure." claudio@