runcon: distinguish runcon specific errors in exit status

runcon: distinguish runcon specific errors in exit status

* src/runcon.c: Use EXIT_CANCELED (125) instead of EXIT_FAILURE (1),
so that errors specific to runcon can be distinguished,
from those of the invoked program.
* doc/coreutils.texi (runcon invocation): Fix the Exit status
description to say we return 125 (not 127) for internal errors.
* tests/misc/runcon-no-reorder.sh: Add a test case.