More user-friendly default result output.

Simplify the default check result output by displaying only the
percentage of used memory; the `-v|--verbose` displays the original
output with detailed values for used/cached/buffers/free memory.

Example:

```
marc@cobalt:~/git/nagios-memory/plugins% ./check_mem -w 80 -c 90 
MEMORY OK - 33% used | used=2762620928;; cached=2460098560;; buffers=106262528;; free=2857504768;;

marc@cobalt:~/git/nagios-memory/plugins% ./check_mem -v -w 80 -c 90
MEMORY OK - 33% used (used: 2688360 kB cached: 2402516 kB buffers: 103788 kB free: 2799952 kB) | used=2752880640;; cached=2460176384;; buffers=106278912;; free=2867150848;;
```