bectl_list: align column output for a mountpoint properly

[rew]

Summary

The output for bectl list is not aligned when the mounted property becomes a long enough
string.

Set the 'Mountpoint' column width based off the boot environment dataset with
the longest mounted property.

This addresses PR: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=241064

Test Plan

All tests passed.

[kevans]

LGTM; much improved.

Approved by: kevans (mentor)