common: allow configvars to be marked dynamic.

To test, we do min-capacity-sat which is simple.  We also update the
listconfigs man page which contained some obsolete information.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

doc/schemas/listconfigs.schema.json

@@ -834,6 +834,13 @@
             "source": {
               "type": "string",
               "description": "source of configuration setting"
+            },
+            "dynamic": {
+              "type": "boolean",
+              "enum": [
+                true
+              ],
+              "description": "Can this be set by setconfig()"
             }
           }
         },