doc: be more consistent when documenting exit status

doc: be more consistent when documenting exit status

* src/system.h (emit_exec_status): A new function to
output standard "Exit status:" info for commands that exec others.
* doc/coreutils.texi (Exit status): Add "ls" and "runcon"
to the list of commands with non standard exit status.
* src/numfmt.c (main): Call initialize_exit_failure() explicitly
to better indicate this utility may exit with something other than
EXIT_FAILURE.
* src/timeout.c (usage): Use more consistent capitalization.
* src/chroot.c: Call emit_exec_status().
* src/env.c: Likewise.
* src/nice.c: Likewise.
* src/nohup.c: Likewise.
* src/runcon.c: Likewise.
* src/stdbuf.c: Likewise.