DERBY-6067: JaCoCo fails with missing permissions when writing results

Added a "jacoco.active" property, which will grant the required privileges to
JaCoCo (or more specifically, all code bases) when set to the empty string ("").
Enabled logic to disable the security manager when spawning processes and at
the same time obtaining code coverage with JaCoCo.
Added comment in build.xml, explaining what derby.tests.jacoco.agent is for.

Patch file: derby-6067-1b-fix_privileges.diff


git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1447388 13f79535-47bb-0310-9956-ffa450edef68