Config changes

- a source's interval must be greater than 0: a source is turned off with
  enabled, not with a zero interval, so JSCSourcesSource rejects an interval of
  0 or less (its validate is promoted to each source's section);
- max_message_size rejects -1: JSCServerWebSocket rejects a max_message_size
  of -1 (util.ByteSizeNoLimit), as a client could then exhaust the server's
  memory;
- every setting documents what 0 or empty means (zero/none, no limit or a
  default).

internal/config/conf_server.go

@@ -7,7 +7,9 @@ import (
 // JSCKServer is the server section's configuration key.
 const JSCKServer string = "server"
 
-// JSCServer configures the outward server.
+// JSCServer configures the outward server. Host is the address to listen
+// on (empty means every interface) and Port the port (0 means any free
+// port).
 type JSCServer struct {
 	Host       string              `cfg:"host"        default:"127.0.0.1"`
 	Port       int                 `cfg:"port"        default:"9393"`