create non-recursive boot environments (bectl)

[rew]

Summary

bectl advertises that it has the ability to create recursive and non-recursive boot environments. This
patch implements that functionality using the be_create_depth API provided by libbe. With this patch,
bectl now works as bectl(8) describes in regards to creating recursive/non-recursive boot environments.

Test Plan

1. Create a non-recursive boot environment and ensure it has no child datasets.
2. Create a recursive boot environment and ensure it has a child dataset.

These tests have been added. The existing tests passed on my machine.

[kevans]

sbin/bectl/bectl.c

227 ↗

(On Diff #57324)

I'd go ahead and combine this branch with the branch below, given how similar they are. The be_snapshot invocation still looks OK even if it's snapname != NULL ? snapname : be_active_path(be), though perhaps a little wordy.