Since opening an encrypted regular file that is locked fails with
ENOKEY, getting the encryption policy of such a file is not possible.
As a result, 'fscrypt status' and 'fscrypt lock' fail on such files.
Provide a better error message that tries to explain what is going on.

Resolves https://github.com/google/fscrypt/issues/393

Currently, fscrypt policies and protectors are world readable, as they
are created with mode 0644.  While this can be nice for use cases where
users share these files, those use cases seem to be quite rare, and it's
not a great default security-wise since it exposes password hashes to
all users.  While fscrypt uses a very strong password hash algorithm, it
would still be best to follow the lead of /etc/shadow and keep this
information non-world-readable.

Therefore, start creating these files with mode 0600.

Of course, if users do actually want to share these files, they have the
option of simply chmod'ing them to a less restrictive mode.  An option
could also be added to make fscrypt use the old mode 0644; however, the
need for that is currently unclear.

Test locking a directory.