postparse now uses tbcmds
sim.tcl is updated to close (and flush) the tbcmds output stream if it
exists.
tb_compat.tcl is the TB version that should be included by newstyle ns scripts.
tb_compat_nop.tcl is the NS version that should be renamed to tb_compat.tcl
and included by scripts when they are run under ns.  I.e. scripts
	source tb_compat.tcl
as the first line and then use either the TB or the NS (nop) version depending
on what they are doing.