bectl(8): don't allow creation of boot environments with spaces

[rew]

Summary

Boot environment datasets that contain spaces are not bootable.

When a user attempts to create a boot environment with a space, abort
the creation and print an error message.

PR: 254441

[allanjude]

If there are other characters that need to be blocked, it might make sense to use strcspn. Although I think in this case, what you have here is fine.