Add conf_enable_app to add items to $enabled_apps, used from conf_enable_xxx functions so one doesn't need to both add stuff to $eanbled_apps and then call conf_enable_xxx, and to avoid enabling the same app twice.

New conf_enable_wiki function to follow the interface of other apps.

apps/blagh/app.rc

@@ -3,6 +3,7 @@ fn conf_enable_blog {
     blagh_dirs=$*
     if(~ $#blagh_dirs 0)
         blagh_dirs=( . )
+    conf_enable_app blagh
 }
 
 fn blagh_init {