EXITFUNC option should be an OptEnum.
Always replace \ with / for Dexter exploit

Fix for the following:
https://github.com/rapid7/metasploit-framework/commit/48199fec271006ed66c4de639cd39e41f05df511#commitcomment-5419010
Land #3011, @wchen-r7's fix for Dexter exploit
Land #3010 - EXITFUNC as OptEnum