uhttpd: added uhttpd.docroot

Passes the document-root to the Lua handler by placing it in uhttpd.docroot.

It could alternatively be placed in env.DOCUMENT_ROOT which would more closely
resemble the CGI protocol; but would mean that it is not available at the time
when the handler-chunk is loaded but rather not until the handler is called,
without any code savings.

Signed-off-by: David Favro <openwrt@meta-dynamic.com>

SVN-Revision: 31571