keyring: support filesystem keyring with v1 encryption policies

keyring: support filesystem keyring with v1 encryption policies

Linux v5.4 and later allows fscrypt keys to be added/removed directly
to/from the filesystem via the new ioctls FS_IOC_ADD_ENCRYPTION_KEY and
FS_IOC_REMOVE_ENCRYPTION_KEY.  Among other benefits, these fix the key
visibility problems that many users have been running into, where system
services and containers can't access encrypted files.

Allow the user to opt-in to using these new ioctls for their existing
encrypted directories by setting in their /etc/fscrypt.conf:

	"use_fs_keyring_for_v1_policies": true

Note that it can't really be on by default, since for v1 policies the
ioctls require root, whereas user keyrings don't.  I.e., setting this to
true means that users will need to use 'sudo fscrypt unlock', not
'fscrypt unlock'.  v2 policies won't have this restriction.