maint: prefer attribute.h in .c files

maint: prefer attribute.h in .c files

This will help us make the transition to C2x, where some
attributes must come at the start of function decls.
Leave the attributes alone in .h files for now,
as the Gnulib tradition is to not expose attribute.h to users.
* bootstrap.conf (gnulib_modules): Add ‘attribute’.
* gl/lib/randperm.c, src/make-prime-list.c, src/system.h:
Include attribute.h.
* gl/lib/strnumcmp.c (strnumcmp): Remove _GL_ATTRIBUTE_PURE here,
as this belongs in the .h file.
* gl/lib/strnumcmp.h (strnumcmp): Add _GL_ATTRIBUTE_PURE here.
* src/sort.c (human_numcompare, numcompare): Now ATTRIBUTE_PURE;
discovered due to strnumcmp.h change.
* gl/lib/randperm.c, src/copy.c, src/dd.c, src/df.c, src/digest.c:
* src/env.c, src/expr.c, src/factor.c, src/ls.c:
* src/make-prime-list.c, src/numfmt.c, src/od.c, src/pathchk.c:
* src/pinky.c, src/pr.c, src/ptx.c, src/realpath.c, src/relpath.c:
* src/seq.c, src/sort.c, src/stat.c, src/stty.c, src/system.h:
* src/tr.c, src/uniq.c, src/wc.c:
In .c files, crefer ATTRIBUTE_CONST to _GL_ATTRIBUTE_CONST, and
similarly for ATTRIBUTE_FORMAT and ATTRIBUTE_PURE.
* src/system.h (FALLTHROUGH): Remove; attribute.h defines it.