If a session is opened for a user twice and the second doesn't have the
AUTHTOK data, pam_fscrypt prints an error message that says it failed to
unlock a protector because AUTHTOK data is missing.  This is misleading
because the protector and its associated policies were already unlocked
by the first session.

To avoid this, move the check for whether the policy is provisioned or
not into policiesUsingProtector().  Also do the same for CloseSession.

Now that it's been requested by users, bring back the "unlock_only"
option, which was originally proposed as part of
https://github.com/google/fscrypt/pull/281 but was dropped in the final
version of that pull request.

Resolves https://github.com/google/fscrypt/issues/357

Update https://github.com/google/fscrypt/issues/350

If someone runs 'passwd USER' as root, the user is assigned a new login
passphrase without their fscrypt login protector being updated.  Detect
this case and show a warning message using pam_info().

Fixes https://github.com/google/fscrypt/issues/273

All pam_fscrypt configuration guides that I'm aware of say to use the
"lock_policies" option for the pam_fscrypt.so session hook.  The
Debian/Ubuntu pam-config-framework config file has it too.

Make locking the default behavior, since this is what everyone wants.

Existing configuration files that contain the "lock_policies" option
will continue to work, but that option won't do anything anymore.

(We could add an option "unlock_only" to restore the old default
behavior, but it's not clear that it would be useful.  So for
simplicity, leave it out for now.)

Configuring whether pam_fscrypt drops caches or not isn't really
something the user should have to do, and it's also irrelevant for v2
encryption policies (the default on newer systems).  It's better to have
pam_fscrypt automatically decide whether it needs to drop caches or not.

Do this by making pam_fscrypt check whether any encryption policy keys
are being removed from a user keyring (rather than from a filesystem
keyring).  If so, it drops caches; otherwise it doesn't.  This
supersedes the "drop_caches" option, which won't do anything anymore.

Allow root to provide the --all-users option to 'fscrypt lock' to force
an encryption key to be removed from the filesystem (i.e., force an
encrypted directory to be locked), even if other users have added it.

To implement this option, we just need to use the
FS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS ioctl rather than
FS_IOC_REMOVE_ENCRYPTION_KEY.

In theory this option could be implemented for the user keyrings case
too, but it would be difficult and the user keyrings are being
deprecated for fscrypt, so don't bother.

Implement adding/removing v2 encryption policy keys to/from the kernel.
The kernel requires that the new ioctls FS_IOC_ADD_ENCRYPTION_KEY and
FS_IOC_REMOVE_ENCRYPTION_KEY be used for this.  Root is not required.

However, non-root support brings an extra complication: the kernel keeps
track of which users have called FS_IOC_ADD_ENCRYPTION_KEY for the same
key.  FS_IOC_REMOVE_ENCRYPTION_KEY only works as one of these users, and
it only removes the calling user's claim to the key; the key is only
truly removed when the last claim is removed.

Implement the following behavior:

- 'fscrypt unlock' and pam_fscrypt add the key for the user, even if
  other user(s) have it added already.  This behavior is needed so that
  another user can't remove the key out from under the user.

- 'fscrypt lock' and pam_fscrypt remove the key for the user.  However,
  if the key wasn't truly removed because other users still have it
  added, 'fscrypt lock' prints a warning.

- 'fscrypt status' shows whether the directory is unlocked for anyone.

FS_IOC_ADD_ENCRYPTION_KEY and FS_IOC_REMOVE_ENCRYPTION_KEY require root
for v1 policy keys, so update the PAM module to re-acquire root
privileges while provisioning/deprovisioning policies that need this.

Also, only set up the user keyring if it will actually be used.

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().