Mitigation for VU#958563: When using a CBC-mode server-to-client cipher

Mitigation for VU#958563:  When using a CBC-mode server-to-client cipher
under SSH-2, don't risk looking at the length field of an incoming packet
until we've successfully MAC'ed the packet.

This requires a change to the MAC mechanics so that we can calculate MACs
incrementally, and output a MAC for the packet so far while still being
able to add more data to the packet later.

[originally from svn r8334]