cmd/utils, rpc/comms: stop XEth when IPC connection ends

There are a bunch of changes required to make this work:

- in miner: allow unregistering agents, fix RemoteAgent.Stop
- in eth/filters: make FilterSystem.Stop not crash
- in rpc/comms: move listen loop to platform-independent code

Fixes #1930. I ran the shell loop there for a few minutes and didn't see
any changes in the memory profile.

miner/miner.go

@@ -133,10 +133,13 @@ func (self *Miner) Register(agent Agent) {
 	if self.Mining() {
 		agent.Start()
 	}
-
 	self.worker.register(agent)
 }
 
+func (self *Miner) Unregister(agent Agent) {
+	self.worker.unregister(agent)
+}
+
 func (self *Miner) Mining() bool {
 	return atomic.LoadInt32(&self.mining) > 0
 }
@@ -146,7 +149,7 @@ func (self *Miner) HashRate() (tot int64) {
 	// do we care this might race? is it worth we're rewriting some
 	// aspects of the worker/locking up agents so we can get an accurate
 	// hashrate?
-	for _, agent := range self.worker.agents {
+	for agent := range self.worker.agents {
 		tot += agent.GetHashRate()
 	}
 	return