PAM: support the authentication facility

Implement the pam_sm_authenticate method, using the noop argument of
lzc_load_key to do a passphrase check without actually loading the key.

This allows using ZFS as the source of truth for user passwords,
without storing any password hashes in /etc or using other PAM modules.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Felix Dörre <felix@dogcraft.de>
Signed-off-by: Val Packett <val@packett.cool>
Closes #14789