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_database.go

@@ -6,7 +6,8 @@ import "reflect"
 const JSCKDatabase string = "database"
 
 // JSCDatabase configures the SQLite database. File is the path to
-// the database file.
+// the database file; empty means none, and commands that need the database
+// fail with a message saying so.
 type JSCDatabase struct {
 	File string `cfg:"file"`
 }