New 'contrib' tool: a script for faking initial KEX.

New 'contrib' tool: a script for faking initial KEX.

encodelib.py is a Python library which implements some handy SSH-2
encoding primitives; samplekex.py uses that to fabricate the start of
an SSH connection, up to the point where key exchange totally fails
its crypto.

The idea is that you adapt samplekex.py to construct initial-kex
sequences with particular properties, in order to test robustness and
security fixes that affect the initial-kex sequence. For example, I
used an adaptation of this to test the Diffie-Hellman range check
that's just gone into 0.64.

(cherry picked from commit 12d5b00d62240d1875be4ac0a6c5d29240696c89)