Patch from Alan Ning (somewhat polished by me): introduce a flag

Patch from Alan Ning (somewhat polished by me): introduce a flag
called 'pending_close'. This deals with the situation in which we're
forwarding a port, have received and locally buffered some data from
the local endpoint but not yet been able to pass it down the SSH
connection due to window limitations, and then the local endpoint
closes its socket. In this situation what we've been doing until now
is to immediately send SSH2_MSG_CHANNEL_CLOSE, causing the data
still in our local buffer to be lost; now we instead set the new
flag, which will remind us to send SSH2_MSG_CHANNEL_CLOSE _after_ we
empty our bufchain.

Should fix at least one manifestation of 'portfwd-close', though I
don't know if it's the cause of all the reports we've ever seen.

[originally from svn r8971]