MFC r346082: libbe(3): use libzfs name validation for datasets/snapshot names

Our home-rolled solution didn't quite capture all of the details, and we
didn't actually validate snapshot names at all. zfs_name_valid captures the
important details, but it doesn't necessarily expose the errors that we're
wanting to see in the be_validate_* functions. Validating lengths
independently, then the names, should make this a non-issue.