libbe(3): restructure be_mount, skip canmount check for BE dataset

Further cleanup after r349380; loader and kernel will both ignore canmount
on the root dataset as well, so we should not be so strict about it when
mounting it. be_mount is restructured to make it more clear that depth==0 is
special, and to not try fetching these properties that we won't care about.

MFC after: 3 days