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_logging.go

@@ -10,7 +10,8 @@ import (
 const JSCKLogging string = "logging"
 
 // JSCLogging configures logging. Level is one of debug, info,
-// warn or error. File, if set, is where commands that log to a file write.
+// warn or error. File, if set, is where commands that log to a file write;
+// empty means they log to the terminal.
 type JSCLogging struct {
 	Level string `cfg:"level" default:"warn"`
 	File  string `cfg:"file"`