pam_fscrypt should never need to do anything for system users, so detect
them early so that we can avoid wasting any resources looking for their
login protector.

If the error is anything other than ErrNotSetup, it might be helpful to
know what is going on.

The metadata validation checks introduced by the previous commits are
good, but to reduce the attack surface it would be much better to avoid
reading and parsing files owned by other users in the first place.

There are some possible use cases for users sharing fscrypt metadata
files, but I think that for the vast majority of users it is unneeded
and just opens up attack surface.  Thus, make fscrypt (and pam_fscrypt)
not process policies or protectors owned by other users by default.
Specifically,

   * If fscrypt or pam_fscrypt is running as a non-root user, only
     policies and protectors owned by the user or by root can be used.

   * If fscrypt is running as root, any policy or protector can be used.
     (This is to match user expectations -- starting a sudo session
     should gain rights, not remove rights.)

   * If pam_fscrypt is running as root, only policies and protectors
     owned by root can be used.  Note that this only applies when the
     root user themselves has an fscrypt login protector, which is rare.

Add an option 'allow_cross_user_metadata' to /etc/fscrypt.conf which
allows restoring the old behavior for anyone who really needs it.

Allow overriding the mountpoint where login protectors are stored by
setting the FSCRYPT_ROOT_MNT environmental variable.  The CLI tests need
this to avoid touching the real "/".

These were found by a combination of manual review and a custom script
that checks for common errors.

Also removed an outdated sentence from the comment for setupBefore().

Running "go vet -shadow ./..." finds all places where a variable might
be incorrectly or unnecessarily shadowed. This fixes some of them.

Now the offending panic will just be logged and the module will fail.
This is important as to not crash the login process.