Update from Gnulib

Update from Gnulib

This includes:
2018-03-07 maint: write-file-hooks -> before-save-hook
2018-03-05 binary-io: pacify gcc -Wunused-parameter
2018-03-05 fflush: adjust to glibc 2.28 libio.h removal
* build-aux/config.guess, build-aux/config.sub:
* build-aux/gitlog-to-changelog, build-aux/install-sh:
* build-aux/move-if-change, build-aux/update-copyright:
* doc/misc/texinfo.tex, lib/binary-io.h, lib/fpending.c:
* lib/stdio-impl.h: Copy from Gnulib.