exit-status: remove ExitStatus typedef

Do not make up our own type for ExitStatus, but use the type used by POSIX for
this, which is "int".  In particular as we never used that type outside of the
definition of exit_status_to_string() where we internally cast the paramter to
(int) every single time we used it.

Hence, let's simplify things, drop the type and use the kernel type directly.