MFC r335844:

MFC r335844:

  core(5): overwrite the oldest core dump

  The '%I' format in the kern.corefile sysctl limits the number of
  core files that a process can generate to the number stored in the
  debug.ncores sysctl. The '%I' format is replaced by the single digit
  index. Previously, if all indexes were taken the kernel would overwrite
  only a core file with the highest index in a filename.
  Currently the system will create a new core file if there is a free
  index or if all slots are taken it will overwrite the oldest one.

  Reviewed by:  kib(code), bcr (updating)
  Differential Revision:        https://reviews.freebsd.org/D15991
  Differential Revision:        https://reviews.freebsd.org/D16084