BF+ENHs: polling backend tracks ino and size now in addition to mtime, filters do not read file unless it has content + few other minor issues

Sorry for a messy PR... if someone feels it would be worth splitting it apart -- might do it.  But outcome should be quite nice:  nearly no sleeps in filters' tests now and everything "just works".
I am not 100% sure if there is no side-effect of tracking not only mtime but also ino and size as to decide either file was modified or not -- but I think it should be just fine.  In turn it allows to resolve racing conditions on file systems with only up-to-a-second resolution for mtime (e.g. ext3 ;-) ).

Please review the diff, and not commits, since they (although atomic) have some changes I reverted later on "in the process".  On the other hand, commits msgs might have more of supporting information... heh heh

Enjoy ;-)