Clear only TCP timeout and Peer Closed Errors

Instead of clearing every error that comes out, clear only the ones that
might be expected from TCP problems from the client side.
Added listening for variable sets over TCP

To allow more fine-grained control than just Mulligans, we want the
ability to ask for new variable values from a remote system.  The first
part of this is implemented in this commit, by making a TCP listening loop
that waits for external connections to request new variable settings.

For more details, see the wiki page
documentation/software/copmutercontrol/setlist/feedback#variables