Anticongestion refinements for ntpd rc script. This reverts r324681

Anticongestion refinements for ntpd rc script. This reverts r324681
and checks if ntp leapfile needs fetching before entering into the
anticongestion sleep.

Unfortunately some ports still use their own sleeps so, this commit
doesn't address the complete problem which is compounded by every
port that uses its own anticongestion mechanism.

Discussed with:		asomers