maint: prefer countof over ARRAY_CARDINALITY

maint: prefer countof over ARRAY_CARDINALITY

* bootstrap.conf (gnulib_modules): Add stdcountof-h.
* src/system.h: Include stdcountof.h.
(ARRAY_CARDINALITY): Remove definition.
* .gitignore (/lib/stdcountof.h): Ignore Gnulib generated file.
* src/csplit.c: Use countof instead of ARRAY_CARDINALITY.
* src/df.c: Likewise.
* src/digest.c: Likewise.
* src/dircolors.c: Likewise.
* src/factor.c: Likewise.
* src/join.c: Likewise.
* src/ls.c: Likewise.
* src/od.c: Likewise.
* src/sort.c: Likewise.
* src/stdbuf.c: Likewise.
* src/tr.c: Likewise.