journal: add one more level on top with AND

When using "-p" and "-b" in combination with "-u", the output is not
what you would expect. The reason is the sd_journal_add_disjunction()
call in add_matches_for_unit() and add_matches_for_user_unit(), which
adds two ORs without taking the other conditions to every OR.

Adding another level on top with AND and sd_journal_add_conjunction()
solves the problem.

Output before:

$ journalctl -o short-monotonic -ab -p 0 -u sshd.service

-- Reboot --
[    3.216305] lenovo systemd[1]: Starting OpenSSH server daemon...
-- Reboot --
[    3.168666] lenovo systemd[1]: Starting OpenSSH server daemon...
[    3.169639] lenovo systemd[1]: Started OpenSSH server daemon.
[36285.635389] lenovo systemd[1]: Stopped OpenSSH server daemon.
-- Reboot --
[   10.838657] lenovo systemd[1]: Starting OpenSSH server daemon...
[   10.913698] lenovo systemd[1]: Started OpenSSH server daemon.
[ 6881.035183] lenovo systemd[1]: Stopped OpenSSH server daemon.
-- Reboot --
[    6.636228] lenovo systemd[1]: Starting OpenSSH server daemon...
[    6.662573] lenovo systemd[1]: Started OpenSSH server daemon.
[    6.681148] lenovo sshd[397]: Server listening on 0.0.0.0 port 22.
[    6.681379] lenovo sshd[397]: Server listening on :: port 22.

As we see, the output is from _every_ boot and priority 0 is not taken
into account.

Output after patch:

$ journalctl -o short-monotonic -ab -p 0 -u sshd.service
-- Logs begin at Sun 2013-02-24 20:54:44 CET, end at Tue 2013-03-19 14:58:21 CET. --

Increasing the priority:

$ journalctl -o short-monotonic -ab -p 6 -u sshd.service
-- Logs begin at Sun 2013-02-24 20:54:44 CET, end at Tue 2013-03-19 14:59:12 CET. --
[    6.636228] lenovo systemd[1]: Starting OpenSSH server daemon...
[    6.662573] lenovo systemd[1]: Started OpenSSH server daemon.
[    6.681148] lenovo sshd[397]: Server listening on 0.0.0.0 port 22.
[    6.681379] lenovo sshd[397]: Server listening on :: port 22.
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"
systemctl: clarify usage of "--all" versus list-unit-files

Novice users might think, that
 $ systemctl --all
is equal to
 $ systemctl list-unit-files

https://bugzilla.redhat.com/show_bug.cgi?id=748512