DERBY-6619 After silently swallowing SecurityExceptions, Derby can leak class loaders

Backported from trunk svn 1619154.
    
Fix to make a path to derby.log OS agnostic; used to fail on Windows.



git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/branches/10.11@1619157 13f79535-47bb-0310-9956-ffa450edef68