Extract the do-while loop in dns__zone_updatesigs() into a separate function

Extract the do-while loop in dns__zone_updatesigs() into a separate function

The do-while loop in dns__zone_updatesigs() is hard to follow due to
heavy nesting and the 'tuple' variable also being used in the outer for
loop.  Add a comment to explain the purpose of the do-while loop.
Extract it into a separate function to decrease indentation and prevent
using 'tuple' in two different loops.