DERBY-6224: Many test failures on latest JDK 8 EA build because of missing SQLPermission

Add the missing permission to the tests' policy files and wrap
AutoloadedDriver's calls to DriverManager.deregisterDriver() in a
privileged block.

Make the deregister=false attribute really prevent
DriverManager.deregisterDriver() from being called during system
shutdown, and make the network server shutdown logic use the attribute
to prevent the need for an extra permission in the default server
policy.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1488125 13f79535-47bb-0310-9956-ffa450edef68