main: Disable enabling both --prune and --txindex

You can have a txindex but with the actual blocks gone, they won't be
much of a help.  Consider allowing these option to be both on in the
future where the txindex is only indexing the non-pruned blocks.

config.go

@@ -1147,6 +1147,14 @@ func loadConfig() (*config, []string, error) {
 		return nil, nil, err
 	}
 
+	if cfg.Prune != 0 && cfg.TxIndex {
+		err := fmt.Errorf("%s: the --prune and --txindex options may "+
+			"not be activated at the same time", funcName)
+		fmt.Fprintln(os.Stderr, err)
+		fmt.Fprintln(os.Stderr, usageMessage)
+		return nil, nil, err
+	}
+
 	// Warn about missing config file only after all other configuration is
 	// done.  This prevents the warning on help messages and invalid
 	// options.  Note this should go directly before the return.