The `socket' function in the backends is only ever checked to see if

it's NULL. Since we already have one back end (uxpty) which doesn't
in fact talk to a network socket, and may well have more soon, I'm
replacing this TCP/IP-centric function with a nice neutral
`connected' function returning a boolean. Nothing else about its
semantics has currently changed.

git-svn-id: svn://svn.tartarus.org/sgt/putty@6810 cda61777-01e9-0310-a592-d414129be87e