Add support for "robots" meta-tag

With this change, cgit will start to generate the "robots" meta-tag, using
a default value of "index, nofollow".

The default value can be modified with a new cgitrc variable, "robots".

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

cgit.h

@@ -139,6 +139,7 @@ extern char *cgit_virtual_root;
 extern char *cgit_script_name;
 extern char *cgit_cache_root;
 extern char *cgit_repo_group;
+extern char *cgit_robots;
 
 extern int cgit_nocache;
 extern int cgit_snapshots;