Improve handling of oobinline sockets; Plink in telnet mode now

doesn't hang when you hit ^C, which is nice. I think a better
solution would involve nonblocking sockets; as it stands it's a
little dependent on what may be quirks of the Linux socket layer.

git-svn-id: svn://svn.tartarus.org/sgt/putty@2175 cda61777-01e9-0310-a592-d414129be87e