hammer2 - SPAN protocol work

hammer2 - SPAN protocol work

* Initial implementation of the LNK_SPAN protocol between two hammer
  service daemons running on different machines.  There's still lots to do.

  mount x P PFSs (P pipes)	(machine A)
	|
  service daemon 		(machine A) (handling P + 1 connections)
	|
  INET SOCKET
	|
  service daemon		(machine B) (handling Q + 1 connections)
	|
  mount x Q PFSs (Q pipes)	(machine B)

* Service deamons starts with LNK_CONN and then interconnect SPANs.

* SPAN protocol allows any number of connections between services daemons and
  from service daemons to physical HAMMER2 mounts.

* Fixed a message write() sequencing bug

* Added some additional debug directives, and also added a remote debug
  directive to connect from one already-running service daemon to another.