Add recursive dataset mounting and unmounting support to pam_zfs_key (#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 <jerzy@kolosowscy.pl>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>