Adjust recovery passphrase generation

Adjust recovery passphrase generation

As per the feedback at https://github.com/google/fscrypt/issues/115
where users didn't understand that the recovery passphrase is important,
restore the original behavior where recovery passphrase generation
happens automatically without a prompt.  This applies to the case where
'fscrypt encrypt' is using a login protector on a non-root filesystem.

However, leave the --no-recovery option so that the recovery passphrase
can still be disabled if the user really wants to.  Also, clarify the
information provided about the recovery passphrase.

Update https://github.com/google/fscrypt/issues/115