hammer2 - Early messaging infrastructure

hammer2 - Early messaging infrastructure

* Implement the core message read/write loop, using poll() and
  non-blocking I/O.  This code can also handle a auxillary events from
  a pipe or tty.

  The initial implementation is very simple and does not yet do message
  tracking and recording.

* Implement piecemeal message parsing and sanity/crc checks.

* Implement piecemeal message generation.

* Implement simple debug messaging for testing and future debugging.

  Debug commands and messages use link-local messages (source=0, target=0),
  in one-way mode (msgid=0), so they're trivial to formulate.

  hammer2 -d node	(run master listener in debug mode)
  hammer2 debug		(in another xterm connect to it)

  Debug messages are really simple, sending a debug command sends a
  line buffer, and the replies contain buffers to write to stdout.

  The target node is responsible for providing the prompt, so we have
  positive feedback that the debug stream is working.