Change protector selection prompts

[josephlr]

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