findmnt: add --tree to allow to enable tree output for --mtab

findmnt: add --tree to allow to enable tree output for --mtab

The --mtab output is merge from kernel and utab on all modern systems
(without classic /etc/mtab). It means we have all necessary information
to generate tree output.

For the backward compatibility --mtab is the list by default, the new
option --tree allows to override the default and enable tree always
when the table contains child-parent relations.

Signed-off-by: Karel Zak <kzak@redhat.com>