hammer2 - cluster / hammer2 service daemon work

hammer2 - cluster / hammer2 service daemon work

* Reformulate the service demon to use the updated libdmsg API

* Add code to track remote block device SPANs and to interface to the
  local /dev/xdisk to create local block devices for the remotes.

• [DBH] sbin/hammer2/cmd_debug.c
• [DBH] sbin/hammer2/cmd_rsa.c
• [DBH] sbin/hammer2/cmd_service.c