[PATCH] cmd: make string/error code more robust against errno leaking

[PATCH] cmd: make string/error code more robust against errno leaking

The i386 sid sbuild fails because apparently some sbuild code
is calling functions that are not implemented so errno is set
during the tests when it is not expected. This leads to test
failures because the die() code will append errno status if
errno is set. This commit fixes this and makes the nightly
test also run on i386.

Gbp-Pq: Name 0012-cherry-pick-pr9844