Fix potential division-by-zero, add optional perfdata via --shortname option

Hi,

this one fixes a potential division by zero error. The commit before that one adds a "--shortname" option which can be used to output simple performance data.

In my perfdata branch HEAD there's also a "check_graphite_multi" version which allows to pull multiple metrics from graphite at once, including perfdata. I use it to get various metrics from graphite into Centreon 2.4 at once. It's sort of a "Graphite is a bitch" approach of integrating it into Centreon...

 ```
./check_graphite_multi -u http://graphite.planethome.com -m 'scale(stats.timers.logstash.prod.weblogic.server01.memory.gc_total_time.upper,1000)|scale(stats.timers.logstash.prod.weblogic.server08.memory.gc_total_time.upper,1000)' -w 10,50 -c 100,200 -s gctime_server01,gctime_server08
WARNING - gctime_server01: 32, gctime_server08: 26 | gctime_server01=32;10;100; gctime_server08=26;50;200;
```

Feel free to cherry pick or anything. Thanks a lot for your (initial) plugin, it helped a lot!