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).

[originally from svn r1198]