SSH2: make it so callback functions can make exec() return early
SSH2: 0x7FFFFFFF -> $this->window_size in one more place
Merge pull request #349 from terrafrost/ssh2-exec-callback-termination

SSH2: make it so callback functions can make exec() return early

* terrafrost/ssh2-exec-callback-termination:
  SSH2: 0x7FFFFFFF -> $this->window_size in one more place
  SSH2: make it so callback functions can make exec() return early