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.