libbe(3): Allow boot environments to be defined on the root of the pool

[jwmaag_gmail.com]

Summary

Also validates the BE root by checking if every child dataset is either
mountpoint '/' or none.

PR: 253445

Test Plan

bectl -r testpool list

[kevans]

Seems fine in principle, I had to do a refresher on whether zfs_iter_filesystems tried to recurse or not but it looks right (non-recursive).

lib/libbe/be.c

176

Space after if, same below and one above here

[jwmaag_gmail.com]

• Style fixes