[MEDIUM] backend: support servers on 0.0.0.0

[MEDIUM] backend: support servers on 0.0.0.0

Till now when a server was configured with address 0.0.0.0, the
connection was forwarded to this address which generally is intercepted
by the system as a local address, so this was completely useless.

One sometimes useful feature for outgoing transparent proxies is to
be able to forward the connection to the same address the client
requested. This patch fixes the meaning of 0.0.0.0 precisely to
ensure that the connection will be forwarded to the initial client's
destination address.