success?()
public
Returns true if stat is successful, false if
not. Returns nil if exited? is not true.
static VALUE
pst_success_p(VALUE st)
{
int status = PST2INT(st);
if (!WIFEXITED(status))
return Qnil;
return WEXITSTATUS(status) == EXIT_SUCCESS ? Qtrue : Qfalse;
}