Improve ntpd offset handling. Call the index of the offset loops

Improve ntpd offset handling.  Call the index of the offset loops
"shift" consistently.  Merge the two offset loops in client_update()
into one.  Use a simple assignment for the best value instead of
memcpy().  Use the same mechanism to loop over the offset array
everywhere to avoid an invalid best value.
tested by weerd@; OK claudio@