cat: replace copy_exact() with write_all

[oech3]

Reduce code

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/tail/retry is no longer failing!
Congrats! The gnu test tests/cut/cut-huge-range is now passing!