Add option to send proxy protocol header

Support for sending a proxy protocol header in Connector-C would be useful for MaxScale and any other client connecting to a server expecting the header. The header contents should be manually set by the client application to have maximal adaptability.

Currently, for example the Percona server does not accept connections without the header if the connection originates from a proxy-enabled address. The ability to "spoof" the header for a normal connection would allow enabling the proxy feature on the server.

For details of the proxy protocol, see http://www.haproxy.org/download/1.8/doc/proxy-protocol.txt.