This patch teaches cgit to expand environment variables in certain
cgitrc option values (cache_root, scan-path, include) plus when
finding the location of cgitrc itself.
One use case for this feature is virtual hosting - e.g. by setting
$CGIT_CONFIG='/etc/cgitrc/$HTTP_HOST' in httpd.conf, all virtual
hosts automatically gets their own cgitrc.
Signed-off-by: Lars Hjemli <hjemli@gmail.com>