Speed up journalctl --system support detection

If journalctl supports --system then abrt process grabs the whole
system journal slowing down the machine significantly.

This checks if help message contains "--system" flag to do this.

Tested on F19:
```
$ time journalctl -h | grep "\-\-system" > /dev/null; echo $?
journalctl -h  0.00s user 0.00s system 82% cpu 0.002 total
grep --color=auto "\-\-system" > /dev/null  0.00s user 0.00s system 71% cpu 0.001 total
0
```
and F18:
```
$ time journalctl -h | grep "\-\-system"; echo $?

real	0m0.003s
user	0m0.001s
sys	0m0.003s
1

```