Teach cgit how to use PATH_INFO

This commit makes cgit use the cgi variables SCRIPT_NAME and PATH_INFO
when virtual-root is unspecified in cgitrc and no url-parameter is
specified on the querystring. This has two nice effects:

* Virtual urls works out of the box, no more need for rewrite-rules in httpd.
* Virtual urls with special querystring characters are handled correctly.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>