[ns] procedures for dynamically changing link delay and bandwidth

[Sally Floyd <floyd@aciri.org>]

I have added a new procedure for dynamically changing link bandwidth,
and have added a new validation test, "./test-all-links", for
illustrating the procedures for dynamically changing link delay
and link bandwidth.  Examples of the commands for changing link
delay and bandwidth are below.  This should all appear in tonight's
snapshot.

- Sally

Changing link delay:
$ns_ at 2.0 "$ns_ delay $node_(r1) $node_(k1) 1000ms duplex"

Changing link bandwidth:
$ns_ at 2.0 "$ns_ bandwidth $node_(r1) $node_(k1) 8Kb duplex"