bectl(8): Fix output of bectl list for the 'Mountpoint' column.

Currently, the output of bectl list doesn't align the 'Mountpoint' column
correctly when the 'mounted' property of a boot environment dataset is longer
than the default column width.

Set the 'Mountpoint' column width to the boot environment dataset with the
longest 'mounted' property or to the default width, whichever is greater.

PR: 241064
Reported by: vermaden@interia.pl
Reviewed by: kevans (mentor)
Approved by: kevans (mentor)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D26048