shred: preserve the ENXIO errno from open

shred: preserve the ENXIO errno from open

This fixes an issue in unreleased commit f77f365ef (shred: don't block
when opening FIFOs with no readers, 2026-05-09).

* src/shred.c: Save the errno before calling stat and use it in the
error message.
* tests/shred/fifo.sh: Call getlimits_ and uses_strace_. Add a test
case.