Protectors can be improperly reverted

[josephlr]

In cmd/fscrypt/commands.go, Protectors should be reverted if they are created. The reversion on failure is here. But this has a problem.

If the protector is selected (not created) in selectOrCreateProtector() the protector will be reverted regardless on any error. This means a failure after selection can result in data loss. The documentation for selectOrCreateProtector() is also wrong, leading to this confusion.