[IZPACK-1118] AntAction listener - add flag "logfile_append" to append to existing Ant action log files

In the built-in Antaction listeners, there is currently the possibility to log to separate logfiles, but each action requires an own log file, because it doesn't append to existing ones.

Add the possibility to choose whether to append to an Ant action log file or not.

Example:
```xml
<antcall order="beforepack" buildresource="service.xml" logfile="${ant.log.file}" logfile_append="true">
	<property name="INSTALL_PATH" value="${INSTALL_PATH}" />
	<property name="service.name" value="${service.name}" />
	<target name="preinstall" />
</antcall>
```