exit-status: kill is_clean_exit_lsb(), move logic to sysv-generator

Let's get rid of is_clean_exit_lsb(), let's move the logic for the special
handling of the two LSB exit codes into the sysv-generator by writing out
appropriate SuccessExitStatus= lines if the LSB header exists. This is not only
semantically more correct, bug also fixes a bug as the code in service.c that
chose between is_clean_exit_lsb() and is_clean_exit() based this check on
whether a native unit files was available for the unit. However, that check was
bogus since a long time, since the SysV generator was introduced and native
SysV script support was removed from PID 1, as in that case a unit file always
existed.

src/systemctl/systemctl.c

@@ -3936,7 +3936,7 @@ static void print_status_info(
                 argv = strv_join(p->argv, " ");
                 printf("  Process: "PID_FMT" %s=%s ", p->pid, p->name, strna(argv));
 
-                good = is_clean_exit_lsb(p->code, p->status, NULL);
+                good = is_clean_exit(p->code, p->status, NULL);
                 if (!good) {
                         on = ansi_highlight_red();
                         off = ansi_normal();