kernel - Fix system lockup w/ kqueue based select/poll due to dup2()

kernel - Fix system lockup w/ kqueue based select/poll due to dup2()

* dup2() was not calling knote_fdclose() on the original descriptor being
  replaced

* fdfree() (called by exit) was not cleaning out knotes either, though
  exit*() probably took care of it already when it closed the descriptors.

* Assert that a file has no knotes on the last fdrop().

Reported-by: everyone + lentferj + alexh + swildner + Studbolt