Created a shiny new abstraction for the socket handling. Has many

Created a shiny new abstraction for the socket handling. Has many
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 r744]