pam: implement a zfs_key pam module

Implements a pam module for automatically loading zfs encryption keys 
for home datasets. The pam module:

  - loads 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 module
    changes the encryption key.

Reviewed-by: Richard Laager <rlaager@wiktel.com>
Reviewed-by: @jengelh <jengelh@inai.de>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Felix Dörre <felix@dogcraft.de>
Closes #9886
Closes #9903

tests/test-runner/bin/zts-report.py

@@ -239,6 +239,7 @@ maybe = {
     'userquota/setup': ['SKIP', exec_reason],
     'vdev_zaps/vdev_zaps_004_pos': ['FAIL', '6935'],
     'zvol/zvol_ENOSPC/zvol_ENOSPC_001_pos': ['FAIL', '5848'],
+    'pam/setup': ['SKIP', "pamtester might be not available"],
 }
 
 if sys.platform.startswith('freebsd'):