Fix issues with the new drinjectlib process wait code

- Put back DR_EXPORT in dr_inject.h
- Fix the NT timeout code to use negative timer units
- Make the Linux impl of dr_inject_process_exit() not block
- Free drinject resources by always calling dr_inject_process_exit()

Review URL: https://codereview.appspot.com/7331051

git-svn-id: https://dynamorio.googlecode.com/svn/trunk@1917 49cc7528-f6fd-11dd-9d1a-b59b2e1864b6