Implement support for the PROXY protocol

[tsipinakis]

Please describe the change you are making

Implement support for the PROXY protocol. Sadly this cannot easily be tested as there are no user-visible changes if this is implemented correctly. (Header is transparently accepted/rejected)

Are you the owner of the code you are sending in, or do you have permission of the owner?

Sent with permission of the owner

The code will be published under the MIT-0 license. Have you read and understood this license?

Yes

[tsipinakis]

@janosdebugs This one is ready for another review as well