Added explicit incoming connection closure on forwarding termination.
Added explicit incoming connection closure on forwarding termination.

This commit adds an additional incoming connection closure in the case
of multiplexer failure on a remote forwarding endpoint. This isn't that
important, because we generally exit the agent process in this case
anyway, but technically the forwarding could be embedded into a larger
process, and while the ForwardAndClose Goroutines would close existing
connections, the new incoming connection would just be left open.

Signed-off-by: Jacob Howard <f862f167b85d41b225785c70d70808bc7337c1fe@havoc.io>

havoc-io/mutagen,havoc-io/mutagen