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.