Append, don't clobber log

Clobbering logs when logrotate is configured to copytruncate results
in null characters in the base log.
See hint in
[[http://serverfault.com/questions/510469/null-bytes-in-a-file-created-by-2-rotated-with-logrotate][Null
bytes in a file created by 2> rotated with logrotate?]]