hooks returning exitcode 0 will:
* run the next hook, if any
* allow the original action, unless successive hook opposes
hooks returning exitcode 100 will:
* abort running successive hooks
* allow the original action
hooks returning anything other than 0 or 100 will:
* abort running successive hooks
* REJECT the original action
zmq can now respond with json; a dict with "rc", "rejectmsg",
"reloc" and so on, just like other hooks replying with json