Define local instances of FALLTHROUGH and UNREACHABLE
requested to merge 3306-undefined-macros-in-contrib-dlz-modules-wildcard-dlz_wildcard_dynamic-c-v9_18 into v9_18
FALLTHOUGH is a copy of how it is defined in <isc/util.h>
UNREACHABLE follows the model used in MacOS /usr/include/c++/v1/cstdlib to determine if __builtin_ureachable is available
(cherry picked from commit 6d68a229)
Closes #3306 (closed)