SSH2: fix possible E_NOTICE
SSH2: Keep track of client -> server window size
SSH2: attempt to handle case where window size is smaller than packet size
SSH2: channel handling adjustments

- keep track of server -> client window size and look out for NET_SSH2_MSG_CHANNEL_WINDOW_ADJUST packets as appropriate
- send client -> server window size packets based on channel of received packet - not of desired channel
- buffer incoming packets based on received packets channel - not on desired channel
Merge branch 'master' of https://github.com/phpseclib/phpseclib