Refactor dns__zone_updatesigs()

The do-while loop in dns__zone_updatesigs(), while short, is hard to follow due to nesting, variable reuse and tricky list traversal. This MR attempts to improve code readability by splitting that loop into a set of short, commented functions (which the compiler should be smart enough to inline anyway, as confirmed with GCC 7.2.0 on x86-64).