Having a discard aclmode ensures that users can "undo" an existing
native ZFS ACL through chmod or setfacl. The ZoL POSIX ACL implementation
writes the POSIX ACL in an xattr separate from the native ZFS ACL and
zfs_acl_chmod_setattr() is used to set POSIX mode during setacl and chmod
requests. If aclmode is set to DISCARD, then existing native ZFS ACL is
replaced with one equivalent to specified mode.
This commit also updates aclmode property names to correct, final ones
returned by openzfs.