dntpd - Improve offset correction precision

dntpd - Improve offset correction precision

* Offset corrections are more accurate with more samples (whereas frequency
  corrections are more accurate when the first and last sample are far apart
  in the time domain).

* Even though dntpd uses a standard deviation test to conditionalize offset
  corrections, it is still possible to get false positives with only 4
  samples.

* Once dntpd has gone into maintainance mode increase the number of
  required samples for an offset correction from 4 to 8.  This makes
  the standard deviation more accurate, reduces false positives, and
  improves stability.

* Retain the original mechanic during aquisition so dntpd applies corrections
  sooner rather than later when first started.