[MINOR] add configuration support for "redir" server keyword

[MINOR] add configuration support for "redir" server keyword

The servers now support the "redir" keyword, making it possible to
return a 302 with the specified prefix in front of the request instead
of connecting to them. This is generally useful for multi-site load
balancing but may also serve in order to achieve very high traffic
rate.

The keyword has only been added to the config parser and to structures,
it's not used yet.