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/runfiles/linux.run

@@ -128,6 +128,10 @@ tags = ['functional', 'mmp']
 tests = ['umount_unlinked_drain']
 tags = ['functional', 'mount']
 
+[tests/functional/pam:Linux]
+tests = ['pam_basic', 'pam_nounmount']
+tags = ['functional', 'pam']
+
 [tests/functional/procfs:Linux]
 tests = ['procfs_list_basic', 'procfs_list_concurrent_readers',
     'procfs_list_stale_read', 'pool_state']