pam: implement a zfs_key pam module

currently the pam module does:
 * load a zfs key and mounts the dataset when a session opens
 * unmounts the dataset and unloads the key when the session closes
 * when the user is logged on and changes the password, the modules
   changes the encryption key.

Signed-off-by: Felix Dörre <felix@dogcraft.de>
Closes #9886