adding configurable AIMD parameters for NS's TCP

[Sally Floyd <floyd@aciri.org>]

I have added configurable AIMD (Additive-Increase Mutiplicative-Decrease)
parameters to the TCP implementation in NS.

For example, the default values of:
    Agent/TCP set increase_num_ 1.00
    Agent/TCP set decrease_num_ 0.5
gives the default additive increase of one packet per RTT, and
decrease by half, while:
    Agent/TCP set increase_num_ 0.41
    Agent/TCP set decrease_num_ 0.75
gives an additive increase of 0.41 packets per RTT, and decreasing
to 3/4 of the old congestion window.

(These two settings for the AIMD parameters happen to have the same
response function in terms of steady-state sending rate as a function
of the packet drop rate.)

The validation test is run by "./test-all-aimd" in tcl/test.

- Sally