connection/channel+session: Allow custom local max packet & window size.

A number of old, buggy (crappy) SSH implementations will cause a channel
open failure if the maximum packet/window size that we sent is too
large for it to handle.

Prior to this patch, the value for the maximum window size and packet
size was hardcoded to 0x20000 and 0x10000 respectively. This patch adds
two new options for setting each respective value, should the need
arise.