Refs #4069: add syslog_include_prefix option to avoid bundle prefix output in syslog

This is a proposed fix for https://cfengine.com/dev/issues/4069.

In 3.5 and previously, reports would be sent to syslog as is, and result in a line such as:
```
Jan 19 02:16:48 hostname cf3[5470]: R: Apache installation checked: OK
```

Since 3.6, the "namespace/bundle path" is prefixed, and the same line is now sent to syslog as:
```
Jan 19 02:16:48 hostname cf3[20151]: /default/test: R: Apache installation checked: OK
```

While I don't mean to discuss here the pros and cons of the new output format (let's keep such discussion elsewhere, this bug is not about that), I would like to state that it is a "bad idea" (TM) to change the format of logs sent via syslog. I know many users that rely on various searches/greps/etc through syslog output from CFEngine to detect changes and errors.

This approach adds a new attribute in ```body common control``` named ```syslog_include_prefix``` to configure this. I have coded it so that it defaults to false, to preserve previous behavior and make the upgrade backwards compatible.

I am of course open to discussion on this code and this approach - I just want to make a positive step forward, so we can hopefully avoid seeing any 3.6.{alpha,beta,rc,final} releases with this bug still in.