Add the NUM_CORE_FILES kernel config option which specifies the limit for the
number of core files allowed by a particular process when using the %I core
file name pattern.

Sanity check at compile time to ensure the value is within the valid range of
0-10.

Reviewed by: jtl, sjg
Approved by: sjg (mentor)
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D6812