Support for UNIX sockets

[yrutschle]

Issue #482 requests support for UNIX sockets. This makes a lot of sense as sslh commonly lives on the same server as the target server, so saving network overhead is easy.

I am planning to do this in three steps:

• Add a is_unix: true setting for protocol entries, which then interprets the host entry as the socket file name. E.g.:
  { name: "http"; is_unix: true; host: "/tmp/ssl_sock"; },
• Add the same setting for listening sockets
• Maybe, later, integrate a URI parser and add a new target_uri setting, which would allow to set all other settings in one string, e.g. "tcp://10.0.0.2:443", "udp://other_host:dns", and based on Nginx' syntax: http://unix:/tmp/mysock.sock. This would auto-fill host, port, is_udp and is_unix settings. I am not sure this is a good idea yet, and am open for feedback :-)