localfs: warn if volume has non-Varasto files

[joonas-fi]

Currently we mount dir like <mountpoint>/varasto-<volume name>.

The varasto-<volume name> dir is the concrete blob database.

We should change the driver to not have mount path like that but only <mountpoint> instead. The driver would then calculate the varasto-<volume name> DB dir itself. This would improve:

• this would enforce the convention
  • I already accidentally mounted the root because I forgot I had to create the subdir
• it's good when you mount a disk to have Varasto files in a subdir and not in root directly
• the driver could have (optional) logic to warn if the disk is "unclean" (= have non-Varasto files)