This is a refactoring step; no behavior changes, and all unit tests pass.
But the way the pivot date used by various calendar functions is set is
changed.  Used to be the pivot-setting operation was snuck in through the
back door using a global pointer to a time-setting function; if the pointer was
NULL various individual functions would call time(NULL). The reason for this
ugly hack was so time could be frozen in test jigs.

The new logic generates a pivot value as far up the call stack as possible
and passes it to where it's needed.  There is one exception to this, in
libntp/prettydate.c, where a module static pivot is set by a new initialization
function; this needs to be called in ntpd's initialization logic.