Prevent connection loops

[yrutschle]

It is possible to make sslh connect to a port it listens to, e.g.:

sslh --listen localhost:443 --ssh localhost:22 --tls localhost:443

This results in creating large amounts of connections to itself. sslh should check no target address is among the listening addresse to address this issue.

[yrutschle]

Code in https://github.com/yrutschle/sslh/pull/69/files can probably be generalied to help solve this.