Log file should honor 'sventon.dir' (Feature Request #4455)

[GoogleCodeExporter]

The log file should go to the location of the 
'sventon.dir' system property.

Don't know if it's possible to do with log4j. 
Currently the config looks like this:

log4j.appender.logfile.File=${java.io.tmpdir}/svent
on_config/sventon.log

System properties can be used, but what if the 
property is not set? Can we fall back on a default 
setting?

Original issue reported on code.google.com by jesper.hammarback on 12 Oct 2011 at 9:11