advantages:
- protocol modules can call sk_write() without having to worry
about writes blocking, because blocking writes are handled in the
abstraction layer and retried later.
- `Lost connection while sending' is a thing of the past.
- <winsock.h> is no longer needed in most modules, because
"putty.h" doesn't have to declare `SOCKET' variables any more,
only the abstracted `Socket' type.
- select()-equivalent between multiple sockets will now be handled
sensibly, which opens the way for things like SSH port
forwarding.
[originally from svn r745]
