conf: Generate and install config sippets for option descriptions

The strongswan.d directory is also created relative to the configured
location of strongswan.conf.

conf/strongswan.conf

@@ -5,8 +5,9 @@
 # Configuration changes should be made in the included files
 
 charon {
+	load_modular = yes
 	plugins {
-
+		include strongswan.d/charon/*.conf
 	}
 }