Add recursive dataset mounting and unmounting support to pam_zfs_key (openzfs#16857)

Introduced functionality to recursively mount datasets with a new
config option `mount_recursively`. Adjusted existing functions to
handle the recursive behavior and added tests to validate the feature.
This enhances support for managing hierarchical ZFS datasets within
a PAM context.

Signed-off-by: Jerzy Kołosowski <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Tony Hutter <[email protected]>

Author: jkolo