Handle HTTP error 511 Network Authentication Required (standard secure proxy authentification/captive portal detection)

["Nicolas Mailhot" <nicolas.mailhot@laposte.net>]

Since
http://code.google.com/p/chromium/issues/detail?id=7338 and
https://bugzilla.mozilla.org/show_bug.cgi?id=479880

there is no clean way for a proxy or captive portal to get a web client to
display an authentication dialog when user credentials expire while he is
browsing on an https url.

(to be sure, the previous methods were insecure and hackish but they existed
because nothing better was available)

The IETF finally set up to fix this problem and defined a standard HTTP error
that lets access control equipments tell the web client authentication or
re-authentication is needed and where the authentication form is located.

http://tools.ietf.org/id/draft-nottingham-http-new-status-04.txt

→ <http://www.rfc-editor.org/queue2.html#draft-nottingham-http-new-status> (the
spec is approved and in the queue for publication as RFC)

Please add error 511 handling in git, so git users that try to access external
git repositories over http can authenticate on the corporate proxy