GitLab

The security checker complains about not using the C11 _s functions.
We don't want to use them, see also
http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1969.htm

* contrib/make-coverage-badge: Add script to create badges
* .gitlab-ci.yml: Create coverage and fuzz-covrage badges
* README.md: Display badges

* gnulib: Update
* .gitlab-ci.yml: Remove --enable-threads=isoc from ./configure

The sanitizer does not work with -O0, changed to -O1.

This only works on a branch called 'coverity-scan'

* configure.ac: Add -Wno-nullability-extension -Wno-nullability-completeness to WARN_CFLAGS
* include/wget/wget.h: Add CLANG_VERSION_AT_LEAST,
  fix attribute defines for clang,
  make use of attributes by using static inline functions for wget allocation,
  define NULLABLE for clang
* libwget/xalloc.c: Rename wget allocation function pointers
* src/options.c: Likewise
* unit-tests/test.c: Likewise

* .gitlab-ci.yml: Create https://gnuwget.gitlab.io/wget2/wget2-latest.tar.gz
* README.md: Instruction to download and build wget2 from tarball