Prevent jsobfu from generating reserved js keywords

While testing BES, I got an error from a script that tried to `var void = 1;`. I can only imagine the lost shells :( This PR adds a check to ensure that duplicate or reserved var names are not generated during obfuscation.

Verification:
- [x] Can run a BES module successfully
- [x] Can run browser_autopwn successfully
- [x] Specs pass