store.

Add test.

Finding the workspace for a given backing store is a common operation
that requires readdir, and should not be expensive.

Further simplifies code.

Deal with nonexistent mountpoints both in mount and umount

* simplifies code

* deal with GetPath() on deleted files

This may occur if we loose connection to the NFS server, for example.

branch do work.

Add test.

Add test.

Introduce new MountOptions struct with AllowOther option.