Minor fixes to drdeploy.c to make it drop-in compatible on Linux

- Adjusts ifdefs to build native drinject without warnings.
- Adds -killpg to kill the child process group after a timeout (i#501).
- Fix the Linux impl of env_var_exists() in dr_config.c
- Fix a bug where an env var isn't present in handle_execve()

Review URL: https://codereview.appspot.com/7307115

git-svn-id: https://dynamorio.googlecode.com/svn/trunk@1915 49cc7528-f6fd-11dd-9d1a-b59b2e1864b6
Merge branch 'master' into drdeploy-killpg