DERBY-5615: Permission problems with classpath subsubprotocol

Wrap CPFile's privileged operations in doPrivileged() so that
classpath databases can be accessed with a security manager.

Make more of the test cases in DatabaseClassLoadingTest and
NativeAuthenticationServiceTest run with a security manager.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1582655 13f79535-47bb-0310-9956-ffa450edef68