add support for (Free)BSD extended attributes

enable with `./configure --with-attr` and `mimetype.use-xattr =
"enable"` in the config.

set attribute with:

    setextattr user Content-Type text/plain path/to/www/file

From: Moritz Wilhelmy <mw@barfooze.de>

git-svn-id: svn://svn.lighttpd.net/lighttpd/branches/lighttpd-1.4.x@2966 152afb58-edef-0310-8abb-c4023f1b3aa9