PAM: add 'forceunmount' flag

Probably not always a good idea, but it's nice to have the option.
It is a workaround for FreeBSD calling the PAM session end earier than
the last process is actually done touching the mount, for example.

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