correctly check for FDE

Soooo this shit has never actually worked correctly.

On a fresh format/install, `diskutil coreStorage info` will fail because the disk hasn't been converted to CS yet. If you enable and then disable FDE, it will continue to return true because at that point the disk _has_ been converted.

Fortunately, Apple anticipated this and gave us the lovely `fdesetup isactive` command, which actually does the right thing. I've verified that it does the right thing on a fresh machine, a machine with FDE enabled, and a machine that has FDE enabled and then disabled. I am very confident in this change. @jbarnette and/or @wfarr please take a quick look.

For what it's worth, `fdesetup` can also be used to enable FDE but that's more than I'm willing to bite off right now.