[kernel] Give scheduler better history tracking

The scheduler again tracks remaining timeslice. Timeslices are bigger,
but once a process uses all of its timeslice, it's demoted and
replenished at the next priority. The scheduler also tracks the last
time a process ran, and promotes it if it's been starved for twice its
full timeslice.

TODO: replenish a small amount of timeslice each time a process is run,
so that more interactive processes keep their priorities.

src/kernel/syscalls/process.cpp

@@ -83,7 +83,7 @@ process_sleep(uint64_t til)
 {
 	auto &s = scheduler::get();
 	auto *p = s.current();
-	log::debug(logs::syscall, "Process %d sleeping until %d", p->pid, til);
+	log::debug(logs::syscall, "Process %d sleeping until %llu", p->pid, til);
 
 	p->wait_on_time(til);
 	s.schedule();