Alternative API for Ocsigen_request.remote_ip

[Julow]

This improve the API for querying the client's connection info, which makes less sense since the added support for unix domain sockets.

The client_conn type no longer loose connection information:

• The client port is available.
• Failure points are removed in favor of a `Unknown case

The `Forwarded_for case makes sure that IP addresses reported through the x_forwarded_for HTTP header can't be used by Accesscontrol by mistake.

There are two API for this information:

• client_conn_to_string: Returns the information in textual form for when the data don't need to be parsed back. For example, ideal in log messages. It's also used to set the x_forwarded_for header, where using a wrong format is better than inventing data in cases where we don't use Internet.
• client_conn: The full data without superfluous transformations.

An other change is that the client IP address doesn't go through getaddrinfo (via Ipaddr_unix.to_inet_addr). This doesn't change anything in the general case but might remove a potential exploit on servers where /etc/hosts is compromised.

This is a second attempt at #270 which was partially reverted because the API name was bad. This new attempt is also free of workarounds and conversions to and from Unix.inet_addr.