Export more functions for initializing and starting node.

For greater flexibility in controlling node's initialization and startup, the
following new functions are exported.

  - node::Init()
  - node::SetupProcessObject()
  - node::Load()
  - node::EmitExit()

These are some of the major steps involved in node::Setup().  Exporting these
functions allows an embedding program to write a replacement for node::Start(),
and to have access to the node process object after it's created.