chroot, env, nice, su: use EXIT_CANCELED for internal failure

chroot, env, nice, su: use EXIT_CANCELED for internal failure

* src/chroot.c (main): Use EXIT_CANCELED, not EXIT_FAILURE.
* src/env.c (main): Likewise.
* src/nice.c (main): Likewise.
* src/su.c (change_identity, main): Likewise.
* doc/coreutils.texi (chroot invocation, env invocation)
(nice invocation, su invocation): Document this.
* NEWS: Likewise.
* tests/misc/invalid-opt (exit_status): Adjust expected results.
* tests/misc/help-version (expected_failure_status): Likewise.