cp,mv: ensure -i,f are not overridden by -u

cp,mv: ensure -i,f are not overridden by -u

Since coreutils 9.3 we had --update={all,older} override -i.
In coreutils 9.5 this was expanded to -u
(to make it consistent with --update=older).

This patch reinstates things so that -i combines with -u instead.
I.e. have -i be protective, rather than selective (like -u).

The -f option of mv is similarly adjusted in this patch,
so now --update does not override any of -f,-i,-n.

* NEWS: Mention the bug fix.
* src/cp.c (main): Don't have -u disable prompting.
* src/mv.c (main): Likewise.
* tests/cp/cp-i.sh: Add a test case for -i.
* tests/mv/update.sh: Likewise.
* tests/mv/i-3.sh. Add a test case for -f.
Fixes https://bugs.gnu.org/70887