systemctl: limit logs in status to current boot

Also reworded a few debug messages for brevity, and added a log
statement which prints out the filter at debug level:

Journal filter: (((UNIT=sys-module-configfs.device AND _PID=1) OR (COREDUMP_UNIT=sys-module-configfs.device AND MESSAGE_ID=fc2e22bc6ee647b6b90729ab34a250b1) OR _SYSTEMD_UNIT=sys-module-configfs.device) AND _BOOT_ID=4e3c518ab0474c12ac8de7896fe6b154)