clock: Improve slaving regression.

clock: Improve slaving regression.

Add domain checks for the input values, and a variable precision
calculation that loops if necessary to ensure we never overflow
accumulators and then silently produce garbage results.

Make the (non-public) linear regression function available for
unit testing by putting it in a separate source file the test
can include. Add a unit test that the new regression function
produces sensible results for several inputs taken from real-world
captures.