PAM: support password changes even when not mounted

There's usually no requirement that a user be logged in for changing
their password, so let's not be surprising here.

We need to use the fetch_lazy mechanism for the old password to avoid
a double prompt for it, so that mechanism is now generalized a bit.

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