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