Suggest a MIN_WINDOW_SIZE and MIN_PACKET_SIZE

Not fully confident with this change, though I will describe my findings
fully in the pull request.  The OpenSSH client requests a maximum packet
size of 16384, but this MIN_PACKET_SIZE value of 32768 causes its request
to be "clamped" up to 32768, later causing an error to stderr on the OpenSSH
client.

Suggest then, to delineate MIN_WINDOW_SIZE from MIN_PACKET_SIZE, as they
are applied.  I don't think there is any minimum value of MIN_PACKET_SIZE,
however we can suggest a value of 4096 for now.