Ronald Landheer-Cieslak points out that the various back ends which

treat all socket closures as clean exits (because the protocol doesn't
provide for transferring a process exit code) could usefully at least
treat _socket errors_ as unclean exits. Patch the Telnet, Rlogin and
Raw backends to retain that information and return INT_MAX to the
frontend.

I wasn't sure whether it was better to solve this by modifying each
affected frontend, or each affected backend. Neither is really ideal;
this is the sort of thing that makes me wish we had a piece of fixed
middleware in between, independent of both platform and protocol.

git-svn-id: svn://svn.tartarus.org/sgt/putty@9730 cda61777-01e9-0310-a592-d414129be87e