zsh-support and more versatile non-interactive execution handling

• shell-integration support of the Z shell for kernel-module- and fanotify
  backend
• tracking of non-interactive commands (e.g. executed via ssh localhost echo foo)
  can now be enabled/disabled on demand for the kernel-module backend.
  The fanotify-backend now double-checks this case for correct usage.
  See README-shell-integration for details
• lots of minor fixes: correct error message for invalid commands on
  shournal -e, preservation of suid-cleared variables for the fanotify-
  shell-integration, deduplicated code for the shell-integration-scripts,
  more meaningful stack traces, etc.
• briefly explain in the README how the backends work and what overhead
  may be expected