More consistent selector for rules

A customer was having problems using policies which had more than one top-level group. The fix is that if there's more than one outer group, then they need to wrap them in one outer element, e.g.:

```xml
<group name="sshd,syslog,">
	<rule id="100000" level="6">
		<match>error: key_read: uudecode</match>
		<description>Wrong or expired ssh keys.</description>
	</rule>
</group>

<group name="local,syslog,">

<!-- Check ntp drift permission file -->
	<rule id="100200" level="5">
		<match>can't open /etc/ntp/drift.TEMP</match>
		<description>Please fix permission.</description>
	</rule>
</group>
```
should be:
```xml
<policy>
	<group name="sshd,syslog,">
		<rule id="100000" level="6">
			<match>error: key_read: uudecode</match>
			<description>Wrong or expired ssh keys.</description>
		</rule>
	</group>

	<group name="local,syslog,">

	<!-- Check ntp drift permission file -->
		<rule id="100200" level="5">
			<match>can't open /etc/ntp/drift.TEMP</match>
			<description>Please fix permission.</description>
		</rule>
	</group>
</policy>
```
However, this requires the selector for the rules to be global, rather than specifically one level down from root.

I also changed the file to overwrite existing converted files, rather than appending.