PAM: add 'recursive_homes' flag to use with 'prop_mountpoint'

It's not always desirable to have a fixed flat homes directory.
With the 'recursive_homes' flag, 'prop_mountpoint' search would
traverse the whole tree starting at 'homes' (which can now be '*'
to mean all pools) to find a dataset with a mountpoint matching
the home directory.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Felix Dörre <felix@dogcraft.de>
Signed-off-by: Val Packett <val@packett.cool>
Closes #14834