snort plugin: Replace the mmap() code with the tail-code from collectd.

snort plugin: Replace the mmap() code with the tail-code from collectd.

This makes following along a file more I/O friendly, I hope. Also, we
don't need to do quite as much buffer processing because lines are
already split for us. Also, in the unlikely case that Snort is writing
the stats file more often than collectd reads it, this will pick up all
intermediate values.