timer: handle fast clocks

timer: handle fast clocks

On a few systems the timer tick can run too fast (eg it's 0.512 of a second
not 0.5) and we can tick ahead of the RTC time. For robustness treat being
a second ahead of locked time as we do being on time - don't tick again so
we'll slide into time.

We shouldn't end up any further out because the big jumps are only when we
have interrupts off for stuff like floppy disk I/O. In that situation we
will always lose time.