timer: add initial support for RTC seconds locking

timer: add initial support for RTC seconds locking

We don't try and use the RTC as our actual clock. Many RTCs are not Y2K safe
or have their own interesting sets of problems. We would also have to keep
doing Unix type v RTC time conversion maths.

Instead we treat the RTC seconds as a free running counter and update our
second tick as we see the RTC one shift. This also gives us up to 59 seconds
of 'overrun' which should do for the worst case disk I/O problems.