like '<numberspace>_<version>', e.g. '1.0_01' for version 01 (used in 1.0 maintenance branch).
this avoids overlap and accidental collisions between different release branches.
this refactors umount code we already used for the testsuite into the platform module's namespace.
also, it exposes that functionality via the cli api, so users can use it via "borg umount <mountpoint>",
which is more consistent than using borg to mount and fusermount -u (or umount) to un-mount.
