Extensive changes that _should_ fix the socket buffering problems,

Extensive changes that _should_ fix the socket buffering problems,
by ceasing to listen on input channels if the corresponding output
channel isn't accepting data. Has had basic check-I-didn't-actually-
break-anything-too-badly testing, but hasn't been genuinely tested
in stress conditions (because concocting stress conditions is non-
trivial).

git-svn-id: svn://svn.tartarus.org/sgt/putty@1198 cda61777-01e9-0310-a592-d414129be87e