cluster - Improve hammer2 connect directive, add /etc/hammer2/autoconn

cluster - Improve hammer2 connect directive, add /etc/hammer2/autoconn

* Improve the hammer2 connect directive, fixing a socket descriptor reuse
  race.

* Monitor the file /etc/hammer2/autoconn which contains a list of hosts
  the hammer2 service demon whould automatically maintain connections to.

* Adjust the dmsg_master_service() API to also allow an alternative
  signalling descriptor and callback to be specified.  This simplifies
  forced connection terminations.

* Fix descriptor leak in dmsg_master_service() on closure.