Merge branch 'master' of ssh://git.code.sf.net/p/nagios/nagioscore

* 'master' of ssh://git.code.sf.net/p/nagios/nagioscore:
  downtime: Ensure that scheduled_downtime_depth never drops below zero
  downtime: Handle error checking in downtime_add()
  downtime: Handle scheduled_downtime_list from downtime_{add,remove}
  downtime: Make the downtime list a doubly-linked one
  downtime: Refactor delete_downtime()
  downtime: Use downtime_compar() when insert-sorting downtime
  downtime: Simplify the insertion sort somewhat
  downtime: Make downtime lookups a 16384 times more efficient
  core: Replace half of xdata/xdddefault.c with a 5-line helper
  common/downtime: Simplify a few functions