Introduce "prefer-language" environment variable,

which allows to influence the negotiation process on request basis
to prefer a certain language, e.g.:

SetEnvIf Request_URI ^/manual/foo/ prefer-language=foo


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@98512 13f79535-47bb-0310-9956-ffa450edef68