[SUREFIRE-946] Allow execution of a callback after the process finished, or the timeout ran up.

The callback is invoked, before calling waitForPumpers, allowing clients to release possible locks on or in the in/out streams.

Patch by Andreas Gudian, applied unmodified

git-svn-id: https://svn.apache.org/repos/asf/maven/shared/trunk@1436551 13f79535-47bb-0310-9956-ffa450edef68