Update from Gnulib

Update from Gnulib

This incorporates:
2019-10-11 Simplify and regularize regex use of ‘assert’
2019-10-09 regex: omit debug assignment when not debugging
2019-10-09 regex: tell compiler there’s at most 256 arcs out
2019-10-09 regex: simplify by assuming C99
2019-10-09 regex: avoid copying of uninitialized storage
2019-09-29 fbufmode: Fix compilation error on glibc >= 2.28 systems
2019-09-28 Update comments that refer to POSIX
2019-09-23 Update URLs and associated text
* doc/misc/texinfo.tex, lib/open.c, lib/regcomp.c:
* lib/regex_internal.c, lib/regex_internal.h, lib/regexec.c:
* lib/stdio-impl.h:
Copy from Gnulib.