Prior to this change the added test fails for me locally at byte
1024. It might depend on some default buffer sizes.

Fixes <https://bugs.gnu.org/35863

>.

* tests/utils.scm ("canonical-newline-port-1024"): Add test.
* guix/utils.scm (canonical-newline-port): Correct comments on CR/LF.
Remove CR even when they're at the end of the buffer.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>