Add a limit to the receive queue length before dropping packets.

Hello,

This patch adds 2 more configuration parameters to the network plugin.
If you have too big queue and want to drop packets, you can configure them.

Extract from new collectd.conf :
```
    QueueLengthLimitLow Number
    QueueLengthLimitHigh Number

When set, if the receive queue length is higher than QueueLengthLimitHigh, the
network plugin will stop dispatching the packets it receives. This mode will
end when the receive queue length returns lower than QueueLengthLimitLow.

If QueueLengthLimitHigh is 0 or not defined, no packet will be dropped.
If QueueLengthLimitHigh is not nul, and QueueLengthLimitLow is nul or
not defined, QueueLengthLimitLow will be forced to some positive value.
```

This is interesting when used with plugin rrdtool_createonly (#243) or other plugins that have a big queue (I/O hell ? issue #75 ?) and may crash. Dropping may be better than losing them because of a crash because you can configure when to drop them.

Regards,
Yves