Added --read-proxy option to require reading the HAProxy protocol line.

[Jimbly]

The read address is then passed to the backend server if --write-proxy or --write-ip is specified. This can be useful if you use HAProxy as your load balancer (in TCP mode) and run stud distributed on each of your servers and want the original client IP address.

[jamwt]

Reviewing, thanks!

[Roguelazer]

This would be extremely nice to have.

[Jimbly]

Though it doesn't work with --write-ip and some other features, the recently merged --proxy-proxy looks like it provides the functionality that I wrote this modification to provide (passing the proxy data from HAProxy to the back-end), so anyone who was waiting for this can probably just use --proxy-proxy instead (depending on what exactly their back-end is).