Fix debugging for forking process run on a remote machine

When remote debugging on a remote machine, pre_child will fail to properly setup the debugger. `TCPSocket.open(acceptor_host, acceptor_port)` will always fail to connect and the remote client will not be able to connect to the new control thread listening on localhost. 

Fix this by: 
* Using `peeraddr` rather than `addr` so that pre_child can connect back to the dispatcher
* Using the `host` arg that rdebug-ide uses rather than 127.0.0.1 as the host so that the client can connect to the newly opened debugger

Hopefully I haven't overlooked something vital, but this change allows me to debug e.g. rails apps when running via unicorn rather than WEBrick/Thin.