Add support for REMOTE_USER authentication

[akrus]

Hello!

It's possible to proxy the username from Apache using REMOTE_USER (or SSL_CLIENT_S_DN_CN when using client certificates) and it would be great if pdnscontrol can support such authentication apart from standard login form. It should be relatively easy, but as I don't have enough experience with Python, I'm not sure which part of source should be corrected. It would be nice if you can point it out or include such feature in future releases.

[zeha]

Ack, but needs explicit opt-in in the config.