maint: update gnulib to latest and adapt streamsavedir usage

* gnulib: Update module to latest.
* gzip.c (treat_dir): Gnulib's streamsavedir API has changed:
call it with a new argument, SAVEDIR_SORT_NONE, to retain the
preceding behavior.
maint: update copyright year range in gzip.texi

* doc/gzip.texi: Update copyright date.
maint: udpate all copyright notices via "make update-copyright"
maint: update .gitignore files

* lib/.gitignore, m4/.gitignore: Adjust to match current sources.
Also, sort.
doc: use UTF-8 in the manual

* doc/gzip.texi: Switch to UTF-8 encoding.  Fix ASCIIisms.
Don't use @sc; plain caps are fine for GNU manuals.
zgrep: exit with status 0 if a file matches and there's no trouble

Reported by Pavel Raiskup in: http://bugs.gnu.org/17760
* zgrep.in (res): Treat exit status 0 to be greater than 1.
Also, exit immediately on software configuration error.