Update from Gnulib

Update from Gnulib

This incorporates:
2020-07-07 dup2: remove support for some very old platforms
2020-07-07 memchr: remove support for some very old platforms
2020-07-04 getumask: new module
2020-07-03 getrandom: fix compilation error on native Windows
2020-07-03 lchmod: simplify after 2020-02-22 change
2020-07-01 manywarnings: improve port to GCC 10.1
2020-06-28 getrandom: fix compilation errors on older versions of mingw
2020-06-29 alloca-opt: fix warning on mingw
* lib/alloca.in.h, lib/dup2.c, lib/getrandom.c, lib/string.in.h:
* lib/sys_stat.in.h, lib/unistd.in.h, m4/dup2.m4, m4/getrandom.m4:
* m4/lchmod.m4, m4/manywarnings.m4, m4/string_h.m4, m4/sys_stat_h.m4:
* m4/unistd_h.m4: Copy from Gnulib.
* lib/gnulib.mk.in, m4/gnulib-comp.m4: Regenerate.