Update to latest gnulib with new copyright year.
Run "make update-copyright" and then...
* tests/init.sh: Sync with gnulib to pick up copyright year.
* bootstrap: Manually update copyright year,
until we fully sync with gnulib at a later stage.
* tests/sample-test: Adjust to use the single most recent year.
* tests/misc/printf-mb.sh: Given we shortcut the single char
(invalid multi-byte) case, add a case to ensure we're correctly
checking the return from mbrtowc().
* src/printf.c (STRTOX): Update to support multi-byte chars.
* tests/misc/printf-mb.sh: Add a new test.
* tests/local.mk: Reference the new test.
* NEWS: Mention the improvement.
Fixes https://bugs.gnu.org/54388
Pádraig Brady