Kernel: Do timekeeping manually instead of asking the RTC all the time.

This introduces a tiny amount of timer drift which I will have to fix
somehow eventually, but it's a huge improvement in timing consistency
as we no longer suddenly jump from e.g 10:45:49.123 to 10:45:50.000.

Kernel/i8253.h

@@ -7,6 +7,7 @@
 namespace PIT {
 
 void initialize();
-dword ticks_since_boot();
+dword ticks_this_second();
+dword seconds_since_boot();
 
 }