printf: with \U, support all valid unicode points

printf: with \U, support all valid unicode points

Previously this was restricted to the C99 universal character subset,
which restricted most values <= 0x9F, as that simplifies the C lexer.
However printf(1) doesn't need this restriction.
Note also the bash builtin printf already supports all values <= 0x9F.

* src/printf.c (main): Relax the restriction on points <= 0x9F.
* doc/coreutils.texi (printf invocation): Adjust description.
* tests/misc/printf-cov.pl: Adjust accordingly.  Add new cases.
* NEWS: Mention the change in behavior.
Reported at https://bugs.debian.org/1022857