Complete rewrite.  Includes a great deal more documentation including a
description of IP allocation.  Also uses the new format.

Note: The section on tb-create-os is unfinished.  I need some info from
Leigh, then I'll fix it up.

tb_compat.tcl

The NOP version of tb_compat.tcl that users should download to run their
scripts with NS.  There is a link to this file in nscommands.html.