journalctl: specify "--unit=" and "--user-unit" multiple times

journalctl: specify "--unit=" and "--user-unit" multiple times

Previously only one "--unit=" or "--user-unit" could be specified.
With this patch, journalcrtl can show multiple units.

$ journalctl -u systemd-udevd.service -u sshd.service -u crond.service -b
-- Logs begin at Sa 2013-03-23 11:08:45 CET, end at Fr 2013-04-12
09:10:22 CEST. --
Apr 12 08:41:37 lenovo systemd[1]: Started udev Kernel Device Manager.
Apr 12 08:41:37 lenovo systemd[1]: Stopped udev Kernel Device Manager.
Apr 12 08:41:38 lenovo systemd[1]: Started udev Kernel Device Manager.
Apr 12 08:41:38 lenovo crond[291]: (CRON) INFO (Syslog will be used
		instead of sendmail.)
Apr 12 08:41:38 lenovo crond[291]: (CRON) INFO (running with inotify
		support)
Apr 12 08:41:39 lenovo systemd[1]: Starting OpenSSH server daemon...
Apr 12 08:41:39 lenovo systemd[1]: Started OpenSSH server daemon.
Apr 12 08:41:39 lenovo sshd[355]: Server listening on 0.0.0.0 port 22.
Apr 12 08:41:39 lenovo sshd[355]: Server listening on :: port 22.
Apr 12 08:41:39 lenovo mtp-probe[373]: checking bus 1, device 8:
"/sys/devices/pci0000:00/0000:00:1a.0/usb1/1-1/1-1.5/1-1.5.6/1-1.5.6.2/1-1.5.6.2.1"