kernel - Change tcp keepalive options from ms to seconds (DISRUPTIVE) (2)

kernel - Change tcp keepalive options from ms to seconds (DISRUPTIVE) (2)

* Refactor the code slightly to adopt the same limits and behavior
  as linux.  Instead of capping out we return EINVAL on any out-of-
  bound value.

* Also note that my history was wrong.  Sephe actually implemented
  these options, not me, and it was in 2011 (8 years ago, not 15+).
  So much for my vague memory!  At the time other operating used wildly
  different metrics, and we chose to use milliseconds.  But in the
  intervening years it looks like the main systems have adopted a
  1-second interval.

  In changing our parameters to match, we avoid a lot of pain when
  porting third party applications that use it, particularly chrome.
  But also a few others.

  If we need sub-second parameters in the future we will implement
  new option keywords for it.