gssh: Return correct value for output stream writes
Add GSSH_DEBUG_LIBSSH environment variable
gssh: Dup fd for libssh

Unfortunately libssh takes ownership of the fd, so we need to dup it.
gssh: Free libssh connection on dispose
gssh: Fix pty size requests

Don't request a *new* pty with a specific size, just change the size
of our current one.