Add text to the troubleshooting chapter about the errors that can be

caused when an active connection times out due to outgoing data
exceeding its maximum number of retries, and mention that this can
occur even when you didn't think you'd sent anything due to rekeys
and/or keepalives.

Unix generates ETIMEDOUT in this situation. Windows, it turns out
after doing an actual experiment by disabling my firewall, generates
ECONNABORTED! So _that's_ what it means under Windows. I wish I'd
done this experiment years ago now.

git-svn-id: svn://svn.tartarus.org/sgt/putty@5585 cda61777-01e9-0310-a592-d414129be87e