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This is a mostly-unmodified copy of the various *_chk implementations and headers from NetBSD, without yet modifying system headers to start actually including them. A future commit will also apply the needed bits to fix ssp/unistd.h. Reviewed by: imp, pauamma_gundo.com (both previous versions), kib Sponsored by: Stormshield Sponsored by: Klara, Inc. Differential Revision: https://reviews.freebsd.org/D32306