Change protector selection prompts

Right now we ask two questions when running "fscrypt encrypt":
1. Do you want a new protector?
2. (If no), which existing protector would you like to use?

We should change this so the initial question is "How do you want to protect your directory":
- Existing Protector 1
- Existing Protector 2
- Create new login protector (only if one doesn't already exist)
- Create new password protector
- Create new keyfile protector

This makes it much easier to figure out what's going on