fix file tracking

[westram]

Broken behavior:

• file tracking does not work on NFS mounts (or similar)
  • inotify events appear for directories, but not for files.
• under osx arb uses polling
  • does not detect deletion and creation of files.

Proposed fix:

• improve polling version to detect modification, deletion and creation of files.
• change use of inotify events:
  • events on files ⇒ instantly trigger poll event for this file
  • events on directories ⇒ call poll events for all tracked files in that directory.
• file tracking callbacks shall only be called from poll events (if conditions are met).

Optimizations:

• it does not make sense to listen to file events, because the poll event for all tracked files in a directory will be triggered by their directory's poll event.