[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/include/log_areas.inc

@@ -1,10 +1,11 @@
 LOG(apic,   info);
-LOG(device, info);
+LOG(device, debug);
 LOG(paging, warn);
 LOG(driver, info);
 LOG(memory, info);
 LOG(fs,     info);
-LOG(task,   info);
+LOG(task,   debug);
+LOG(loader, info);
 LOG(boot,   debug);
 LOG(syscall,debug);
 LOG(vmem,   debug);