MFC r347027-r347028: libbe(3) mountpoint handling fixes

r347027:
libbe(3): Properly mount BEs with mountpoint=none

Instead of pretending to successfully mount them while not actually
mounting anything, we'll now actually mount them *and* claim we mounted them
successfully.

Reported by: ler

r347028:
libbe: set mountpoint=none in be_import

If we're going to set a mountpoint at all, mountpoint=none makes more sense
than mountpoint=/.