guardian: Use Inotify for detecting file modifications.

guardian: Use Inotify for detecting file modifications.

If a monitored file gets modified, guardian now recognizes this by
using the inotify interface provided from the Linux kernel and stores
this information in a processing queue.

This is a more efficent way than the old one, which periodly checked if the
filesize had been changed.

Also doing some more code cleanup and add input validation.