Fix contributed by Brian. The general idea is to allow global

variables to be renamed from within a subroutine:

	proc create_testnet {} {
		set s1 [$ns node]
 	}

Chris' redefined set did not allow this. This is further complicated
by the fact that its difficult to tell whats a global variable. You
can declare a variable global, but until its set for the first time,
the TCL info function will not tell you its a global. We do not want
to rename locals of course, since that would make no sense to do.
Anyway, Brian contributed the code to fix the new set function, and I
looked at it and did my best to understand it (I kinda do), but no way
I can be sure! I ran it through the test suite and spot checked the DB
state. Seems to work okay.