action pipe

This is a proposal of new Knot feature: a system of (named) pipes, as a substitute for direct calling of user scripts from knotd process.

The user might want to call a script when an event occurs. First motivation for this is publishing CDS/CDNSKEY records - the user shall react with updating DS record at the parent zone.

The configuration might look like follows:

event:
  - fifo: /my/scripts/knot_hok_ds.fifo
    action: publish_ds

and the user script like this:

cat /my/scripts/knot_hok_ds.fifo | while read x; do
  perform_action "$x"
done

Some notes to this solution:

• this avoids system() or popen() calls by knotd which are known to be very evil
• this avoids active waiting at the script side (would be harmful if many zones)
• O_NONBLOCK has to be used at knotd side. If no script is listening on the configured fifo, knot just logs a warning
• just one script can listen at a fifo. However, it is possible to configure more fifos for any set of possible event actions

Some suggestions:

• the emitted action request consists of a single line (terminated with \n), with fields separated by colon :, fields are: zone, action type, data ... do we need a timestamp?? can colon conflict with zone name or data???