Re-enable 'index-info' and add support for 'root-desc' in cgitrc

The 'index-info' option got lost when the layout was converted from
sidebar to old-fashioned header (noticed by Harley Laue, thanks!), and
this commit re-enables it.

But there is now also an alternative in the 'root-desc' option; where
'index-info' specifies a file to include, 'root-desc' specifies the text
literally. This might be nicer for the one-liner descriptions which these
options typically provides.

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

cgit.h

@@ -132,6 +132,7 @@ struct cgit_config {
 	char *repo_group;
 	char *robots;
 	char *root_title;
+	char *root_desc;
 	char *script_name;
 	char *virtual_root;
 	int cache_dynamic_ttl;