This PR moves the userland proxies for TCP and UDP traffic out of the
main docker daemon's process ( from goroutines per proxy ) to be a
separate reexec of the docker binary. This reduces the cpu and memory
needed by the daemon and if the proxy processes crash for some reason
the daemon is unaffected. This also displays in the standard process
tree so that a user can clearly see if there is a userland proxy that is
bound to a certain ip and port.
```bash
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
5d349506feb6 busybox:buildroot-2014.02 "sh" 13 minutes ago Up 1 seconds 0.0.0.0:49153->81/tcp, 0.0.0.0:49154->90/tcp hungry_pike
root@1cbfdcedc5a7:/go/src/github.com/docker/docker# ps aux
USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
root 1 0.0 0.1 18168 3100 ? Ss 21:09 0:00 bash
root 8328 0.7 0.6 329072 13420 ? Sl 22:03 0:00 docker -d -s vfs
root 8373 1.0 0.5 196500 10548 ? Sl 22:03 0:00 userland-proxy -proto tcp -host-ip 0.0.0.0 -host-port 49153 -container-ip 10.0.0.2 -container-port 81
root 8382 1.0 0.5 270232 10576 ? Sl 22:03 0:00 userland-proxy -proto tcp -host-ip 0.0.0.0 -host-port 49154 -container-ip 10.0.0.2 -container-port 90
root 8385 1.2 0.0 3168 184 pts/0 Ss+ 22:03 0:00 sh
root 8408 0.0 0.1 15568 2112 ? R+ 22:03 0:00 ps aux
```
This also helps us to cleanly cleanup the proxy processes by stopping
these commands instead of trying to terminate a goroutine.
Signed-off-by: Michael Crosby <michael@docker.com>
